System and method for facet joint replacement

ABSTRACT

A system for replacing at least a portion of a natural facet joint includes a fixation member implantable in a vertebra, an inferior facet articular surface and an inferior strut which may be formed separately from the inferior articular surface. The inferior strut has a first end securable to the fixation member and a second end which may comprise a sphere with a hemispherical surface. An attachment mechanism may include a capture feature shaped to receive the second end of the inferior strut, and the mechanism may provide an adjustable configuration, allowing polyaxial adjustment between the inferior articular surface and the second end. A locking member may be actuated to exert force on the second end to provide a locked configuration. The system may further include a superior facet joint implant with a superior articular surface shaped to articulate with the inferior articular surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of each of the following:

pending U.S. application Ser. No. 12/104,726, filed Apr. 17, 2008, whichcarries Applicants' docket no. FSI-20, and is entitled FACET JOINTREPLACMENT; and

pending U.S. application Ser. No. 12/104,855, filed Apr. 17, 2008, whichcarries Applicants' docket no. FSI-30, and is entitled FACET JOINTREPLACMENT. Each of these is a continuation-in-part of the following:

pending U.S. application Ser. No. 11/972,158, filed Jan. 10, 2008, whichcarries Applicants' docket no. FSI-19, and is entitled TAPER-LOCKINGFIXATION SYSTEM, which claims the benefit of the following:

pending U.S. Provisional Patent Application No. 60/884,233, filed Jan.10, 2007, which carries Applicants' docket no. FSI-19 PROV, and isentitled TAPER-LOCKING ROD FIXATION SYSTEM;

pending U.S. Provisional Application No. 60/912,323, filed Apr. 17,2007, which carries Applicants' docket no. FSI-20 PROV, and is entitledAFRS MULTI-LEVEL IMPLANT SYSTEM;

pending U.S. Provisional Application No. 60/950,012, filed Jul. 16,2007, which carries Applicants' docket no. FSI-22 PROV, and is entitledINFERIOR FACET IMPLANT HOLDER;

pending U.S. Provisional Application No. 60/950,021, filed Jul. 16,2007, which carries Applicants' docket no. FSI-23 PROV, and is entitledMONORAIL INSTRUMENT GUIDANCE SYSTEM FOR LUMBAR SPINAL SURGERY;

pending U.S. Provisional Application No. 60/950,031, filed Jul. 16,2007, which carries Applicants' docket no. FSI-24 PROV, and is entitledLINEAR POLYAXIAL LOCKING MECHANISM WITH TOOL;

pending U.S. Provisional Application No. 60/950,038, filed Jul. 16,2007, which carries Applicants' docket no. FSI-25 PROV, and is entitledMOBILE INFERIOR FACET BEARING WITH SUPERIOR CLIP;

pending U.S. Provisional Application No. 60/957,505, filed Aug. 23,2007, which carries Applicants' docket no. FSI-26 PROV, and is entitledDYNAMIC STABILIZATION AND STATIC FIXATIO OPTIONS FOR FACET REPLACEMENTPROSTHESIS;

pending U.S. Provisional Application No. 60/968,324, filed Aug. 27,2007, which carries Applicants' docket no. FSI-21 PROV, and is entitledINTERVERTEBRAL DISC IMPLANT WITH FACET MOTION CONSTRAINTS;

pending U.S. Provisional Application No. 60/968,925, filed Aug. 30,2007, which carries Applicants' docket no. FSI-27 PROV, and is entitledSYSTEMS AND METHODS FOR LESS INVASIZE FACET JOINT REPLACEMENT;

pending U.S. Provisional Application No. 60/975,731, filed Sep. 28,2007, which carries Applicants' docket no. FSI-30 PROV, and is entitledMONOLITHIC INFERIOR IMPLANT STRUT WITH INTEGRAL CROSS LINK CLAMP;

pending U.S. Provisional Application No. 60/984,434, filed Nov. 1, 2007,which carries Applicants' docket no. FSI-34 PROV, and is entitledSUPERIOR INSTRUMENTS;

pending U.S. Provisional Application No. 60/984,428, filed Nov. 1, 2007,which carries Applicants' docket no. FSI-35 PROV, and is entitled CROSSLINK CLAMP;

pending U.S. Provisional Application No. 60/984,594, filed Nov. 1, 2007,which carries Applicants' docket no. FSI-36 PROV, and is entitled LOWPROFILE POLYAXIAL FACET IMPLANT;

pending U.S. Provisional Application No. 60/984,798, filed Nov. 2, 2007,which carries Applicants' docket no. FSI-31 PROV, and is entitled LOWPROFILE POLYAXIAL FACET IMPLANT;

pending U.S. Provisional Application No. 60/984,814, filed Nov. 2, 2007,which carries Applicants' docket no. FSI-32 PROV, and is entitled HINGEDEYELET SCREW;

pending U.S. Provisional Application No. 60/984,983, filed Nov. 2, 2007,which carries Applicants' docket no. FSI-33 PROV, and is entitledADJUSTABLE FACET IMPLANT BASE PIECE;

pending U.S. Provisional Application No. 61/014,344, filed Dec. 17,2007, which carries Applicants' docket no. FSI-37 PROV, and is entitledINFERIOR STRUT UPDATE;

pending U.S. Provisional Application No. 61/015,866, filed Dec. 21,2007, which carries Applicants' docket no. FSI-28 PROV, and is entitledINTERVERTEBRAL DISC IMPLANT WITH FACET MOTION CONSTRAINTS INCLUDINGPOSTERIOR COMBINATION;

pending U.S. Provisional Application No. 61/015,876, filed Dec. 21,2007, which carries Applicants' docket no. FSI-29 PROV, and is entitledINTERVERTEBRAL DISC IMPLANT WITH FACET MOTION CONSTRAINTS AND METHODSFOR IMPLANT ALIGNMENT;

pending U.S. Provisional Application No. 61/015,886, filed Dec. 21,2007, which carries Applicants' docket no. FSI-38 PROV, and is entitledEYELET PEDICLE SCREW WITH MULTI-AXIAL FIXATION; and

pending U.S. Provisional Application No. 61/015,840, filed Dec. 21,2007, which carries Applicants' docket no. FSI-39 PROV, and is entitledCERVICAL PLATE WITH FACET MOTION CONTROL.

This application also claims the benefit of the following, which areincorporated herein by reference:

pending U.S. Provisional Application No. 61/023,927, filed Jan. 28,2008, which carries Applicants' docket no. FSI-40 PROV, and is entitledAFRS GENERATION II INSTRUMENTS;

pending U.S. Provisional Application No. 61/033,473, filed Mar. 4, 2008,which carries Applicants' docket no. FSI-41 PROV, and is entitled TOPLOADING RECEIVER FOR AN ADJUSTABLE FACET REPLACEMENT;

pending U.S. Provisional Application No. 61/040,041, filed Mar. 27,2008, which carries Applicants' docket no. FSI-42 PROV, and is entitledFACET JOINT REPLACEMENT;

pending U.S. Provisional Application No. 61/042,896, filed Apr. 7, 2008,which carries Applicants' docket no. FSI-43 PROV, and is entitled SPINALFIXATION ON AN IMPLANT BASE; and

pending U.S. Provisional Application No. 61/045,526, filed Apr. 16,2008, which carries Applicants' docket no. FSI-44 PROV, and is entitledINFERIOR BASE-SPLIT CLAMP AND MULTI-LEVEL SPLIT CLAMP.

All of the foregoing are incorporated herein by reference.

BACKGROUND OF THE INVENTION The Field of the Invention

The invention relates to spinal surgery. More specifically, theinvention relates to replacement of natural vertebral facet joints withimplantable artificial facet joint replacements.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present invention will now be discussed withreference to the appended drawings. It is appreciated that thesedrawings depict only typical embodiments of the invention and aretherefore not to be considered limiting of its scope.

FIG. 1 is a perspective view of a portion of a spine with a bi-lateralfacet joint replacement system implanted into two adjacent vertebrae;

FIG. 2 is perspective view of an inferior facet joint implant coupled toa crosslink rod;

FIG. 3 is an exploded view of the inferior facet joint implant andcrosslink rod of FIG. 2;

FIG. 4 is a partial cross-sectional view of an attachment mechanism ofthe facet joint implant of FIG. 2;

FIG. 5 is a perspective view of a fixation assembly secured to aninferior strut;

FIG. 6 is an exploded view of the fixation assembly and inferior strutof FIG. 5;

FIG. 7 is a perspective view of a fixation assembly secured to asuperior facet joint implant;

FIG. 8 is a perspective view of an alternate fixation assembly securedto a superior facet joint implant;

FIG. 9 is an exploded view of the alternate fixation assembly andsuperior facet joint implant of FIG. 8;

FIG. 10 is a partial cross-sectional view of the alternate fixationassembly and superior facet joint implant of FIG. 8;

FIG. 11 is a perspective view of a portion of a spine with an alternatebi-lateral facet joint replacement system implanted into two adjacentvertebrae;

FIG. 12 is a perspective view of an inferior facet joint implant coupledto a crosslink rod;

FIG. 13 is an exploded view of the inferior facet joint implant andcrosslink rod of FIG. 12;

FIG. 14 is a partial cross-sectional view of the inferior facet jointimplant of FIG. 12;

FIG. 15 is a perspective view of a fixation assembly and an inferiorstrut;

FIG. 16 is an exploded view of the fixation assembly and inferior strutof FIG. 15;

FIG. 17 is a perspective view of an inferior implant body coupled to aclip;

FIG. 18A is a perspective view of an alternate inferior strut; and FIG.18B is a perspective view from an alternate angle of the strut of FIG.18A;

FIG. 19 is a perspective view of the clip of FIG. 17 and a plug;

FIG. 20 is a perspective view of the clip of FIG. 17 coupled to aninferior facet joint implant, and the superior facet joint implant andfixation assembly of FIG. 8;

FIG. 21 is a perspective view of the inferior and superior facet jointimplants of FIG. 20 joined by the clip of FIG. 17;

FIG. 22 is a perspective view of a multi-level facet joint replacementsystem implanted in a portion of a spine;

FIG. 23 is a lateral perspective view of a portion of the multi-levelfacet joint replacement system of FIG. 22;

FIG. 24 is a perspective view of a fixation assembly of FIG. 22;

FIG. 25 is a perspective view of an inferior facet joint implant of FIG.22;

FIG. 26A is a perspective view of a superior facet joint implant of FIG.22; and FIG. 26B is a perspective view of an alternate embodimentsuperior facet joint implant of FIG. 22;

FIG. 27A is a lateral view of a fixation assembly base member; FIG. 27Bis a posterior view of the fixation assembly base member of FIG. 27A;FIG. 27C is an anterior perspective view of the fixation assembly basemember of FIG. 27A; and FIG. 27D is a cross-sectional view of thefixation assembly base member of FIG. 27A;

FIG. 28A is a lateral perspective view of an alternate fixation assemblybase member; and FIG. 28B is a cross-sectional view of the fixationassembly base member of FIG. 28A;

FIG. 29 is a lateral perspective view of an alternate fixation assemblybase member;

FIG. 30 is a lateral perspective view of an alternate fixation assemblybase member;

FIG. 31 is a lateral perspective view of an alternate fixation assemblybase member;

FIG. 32 is a lateral perspective view of an alternate fixation assemblybase member;

FIG. 33 is a lateral perspective view of an alternate fixation assemblybase member;

FIG. 34 is a lateral perspective view of an alternate fixation assemblybase member;

FIG. 35 is a posterior perspective view of a bi-lateral low-profilefacet joint replacement system implanted into two adjacent vertebrae;

FIG. 36A is a perspective view of the low profile inferior facet implantof FIG. 35; and FIG. 36B is an alternate perspective view of the lowprofile inferior facet implant of FIG. 35;

FIG. 37 is an exploded view of the low profile inferior facet implant ofFIG. 35;

FIG. 38 is a perspective view of a fixation assembly of FIG. 35;

FIG. 39 is a perspective view of another fixation assembly of FIG. 35;

FIG. 40 is an exploded view of the fixation assembly of FIG. 39;

FIG. 41 is a cross-sectional view of the fixation assembly of FIG. 39;

FIG. 42 is a perspective view of an alternate fixation assembly;

FIG. 43 is an exploded view of the alternate fixation assembly of FIG.42;

FIG. 44 is a partially exploded perspective view of superior andinferior facet joint implants coupled to the clip of FIG. 17;

FIG. 45A is a perspective view of an alternate embodiment of a clip; andFIG. 45B is a perspective view of the clip of 45A coupled to an inferiorfacet joint implant;

FIG. 46 is a perspective view of the clip and implant of FIG. 45 coupledto a delivery tool, and a superior facet joint implant;

FIG. 47 is a perspective view of the clip and implant of FIG. 45 coupledto a flexing tool, coupled to a superior facet joint implant;

FIG. 48 is a perspective posterior view of a bi-lateral facet jointreplacement system with medial-lateral adjustability implant in aportion of a spine;

FIG. 49 is a caudal perspective view of a portion of the system of FIG.48;

FIG. 50 is an exploded perspective view of a portion of the system ofFIG. 48; and

FIG. 51 is a caudal partial cross-sectional view of a portion of thesystem of FIG. 48.

DETAILED DESCRIPTION

The present invention advances the state of the art by providing systemsand methods that can be used to replace natural vertebral facet jointswith implantable artificial facet joint prostheses in a manner thatprovides a high degree of implant adjustability, simplicity, and ease ofuse.

In this application, “polyaxial” rotation is rotation that can occurabout at least two axes that are not parallel to each other. “Lock-out”or “lock-down” between two or more component parts refers to a state inwhich movement of any component part is prevented by frictional,compression, expansion, or other forces. A “taper-lock connector” refersto a locking mechanism that uses a taper to effect locking.

Referring to FIG. 1, a perspective view depicts a portion of a spineincluding a first vertebra 2 and a second vertebra 4. A system 10 ofbi-lateral facet joint replacements joined by a crosslink rod passingthrough a spinous process 6 is implanted in the vertebrae. On the leftside of the vertebrae, an inferior facet joint implant 100 is secured toa fixation assembly 300 implanted in vertebra 4. Together the inferiorfacet joint implant 100 and fixation assembly 300 form inferior facetjoint prosthesis 11. A superior facet joint implant 200 is secured to afixation assembly 300 implanted in vertebra 2, and together the superiorfacet joint implant 200 and fixation assembly 300 form superior facetjoint prosthesis 12. On the right side of the vertebrae, an inferiorfacet joint implant 101 is secured to a fixation assembly 300 implantedin vertebra 4, and a superior facet joint implant 201 is secured to afixation assembly 200 implant in vertebra 2. It is appreciated that manyof the facet joint replacement protheses, implants and fixationassemblies described herein may each be configured in a “right” or a“left” configuration to be implanted on the right or left lateral sideof the vertebrae. However, in most cases, only one (right or left)configuration will be described, and it is assumed that the other (rightor left) configuration is a mirror-image of the one described. It isalso appreciated that the implants described herein may be implantedbi-laterally as in FIG. 1, or unilaterally, if desired.

Referring to FIG. 2, a perspective view depicts polyaxially adjustableleft inferior facet joint implant 100. Inferior facet joint implant 100comprises an inferior articular body 102, an inferior strut 104, and anattachment mechanism 106 which adjustably secures the articular body tothe inferior strut. The attachment mechanism 106 has an adjustableconfiguration in which the inferior articular body 102 can rotaterelative to the inferior strut 104 about three orthogonal axes, and ithas a locked configuration in which the inferior articular body 102 isrigidly secured to inferior strut 104. A crosslink rod 108 mayoptionally be secured to the implant 100 by a split clamp 110. Theattachment mechanism 106 may be actuated to simultaneously lock thecrosslink rod 108 in the split clamp 110 as the inferior articular body102 is locked to the inferior strut 104. A clamp axis 111 extendslongitudinally through the attachment mechanism. A strut axis 105extends longitudinally along the inferior strut 104.

Referring to FIG. 3, an exploded perspective view illustrates thecomponent parts which may comprise the left inferior facet joint implant100. The inferior articular body 102 is shell-like and has asubstantially concave interior cavity 112 which is defined by aninterior wall 114. A first chamfered opening 116 and a second chamferedopening 118 in the inferior articular body 102 create a passagewaythrough which a portion of the inferior strut may fit when the implantis assembled. An attachment post opening 120, which may also bechamfered, is situated orthogonal to the first and second chamferedopenings 116, 118. The chamfered openings may provide additional rangeof motion between the inferior articular body and the inferior strut 104as the articular body 102 is polyaxially adjusted prior to locking down.An inferior articular surface 122 is located on the exterior of theinferior articular body 102, and is shaped to replace a natural inferiorarticular surface of a vertebra. Inferior facet implant 100 may beimplanted in conjunction with a superior facet implant, wherein theinferior articular surface 122 articulates with an artificial superiorfacet articular surface. Alternately, inferior facet implant 100 may beimplanted such that the inferior articular surface 122 articulates witha natural superior facet articular surface. In either case, thearticulation between superior and inferior articular surfaces, whethernatural or artificial, provides preservation of a level of naturalspinal motion.

FIG. 4 displays the attachment mechanism in a cross-sectional view. Theattachment mechanism 106 is configured to provide polyaxialadjustability between the inferior articular surface 122 and theinferior strut 104. Once the desired orientation of the articularsurface 122 relative to the inferior strut 104 is reached, theattachment mechanism 106 may be locked down, securing the articularsurface to the inferior strut. Referring to FIGS. 3 and 4, theattachment mechanism comprises a locking member which is a threadedconical expander 126, an expandable member which is an expandable splitshell 128, the split clamp 110, and a nut 130. An alternative embodimentof an attachment mechanism may exclude the split clamp 110.

The split shell 128 has a circular neck portion 132 through which passesa bore 134. The bore opening is surrounded by a radial spline 136.Adjacent to the neck portion 132 is a spherical portion 138 whichcomprises two expandable lobes 140, 142. An interior surface 143 of thelobes 140 may be tapered. The present embodiment of the inventionincludes two lobes, however it is appreciated that more lobes may beincluded, or other expandable portions, in other embodiments. The splitshell 128 fits over the conical expander 126 such that a threaded post146 of the conical expander passes through the bore 134. An expansionportion 148 of the conical expander 126 is forked and has two opposingflanges 150, 152 which are shaped to fit around and grip the inferiorstrut 104. An inner wall 153 of the flanges is curved to fit around theinferior strut, and the outer walls 154, 156 are tapered.

The split ring clamp 110 comprises an inner ring 160, an outer ring 162and a collar 164 which joins the inner and outer rings. The collar 164is shaped to receive and grip the crosslink rod 108. The split ringclamp is configured such that when the inner and outer rings 160, 162are compressed together, a diameter of the collar 164 decreases and thecollar can tighten around and secure the crosslink rod. The surface ofan exterior side of the inner ring 160 is a radial spline 166, which isshaped to engage with the radial spline 136 on the split shell 128.

When assembled, the split shell 128 fits over the conical expander 126,and the two parts fit within the inferior articular body 102 such thatthe interior cavity 112 houses the expansion portion 148 of the conicalexpander 126 nested inside the spherical portion 138 of the split shell128. The conical expander 126, split shell 128 and inferior articularbody 102 are oriented so that in general the flanges 150, 152 areadjacent to the lobes 140, 142, and the lobes are adjacent to theinterior wall 114 of the interior cavity 112. A rod portion of theinferior strut 104 fits between the flanges 150, 152 of the conicalexpander.

The split ring clamp 110 fits over the threaded post 146 of the conicalexpander so that the radial spline 166 of the split clamp meets theradial spline 136 of the split shell 128. The crosslink rod 108 extendsthrough the collar 164 of the split clamp. The nut 130 is threaded ontothe threaded post 146 of the conical expander.

Until the attachment mechanism 106 is locked down by actuating the nut130, the implant is adjustable in multiple ways. The crosslink rod 108has relative angular freedom of motion about the clamp axis 111 and theinferior strut axis 105. The position of the crosslink rod 108 relativeto the split clamp 110 may be adjusted such that a relatively longer orshorter portion of the crosslink rod 108 extends through the clamp. Thisprovides an opportunity to select the best fit to the patient's anatomyand the specific vertebral level being treated. Similarly, the positionof the inferior strut 104 may be adjusted relative to the inferiorarticular body 102 such that a relatively longer or shorter length ofthe inferior strut 104 extends through the flanges 150, 152 of theconical expander 126. Also, the inferior strut 104 has relative angularfreedom of motion about the clamp axis 111. The inferior articular body102 may be polyaxially rotated about the conical expander 126 and thesplit shell 128. The adjustments provide relative rotation between theinferior articulation surface 122 and the inferior strut 104 about threeorthogonal axes. In addition, prior to lockdown, relative translationbetween the inferior strut 104, the inferior articulation surface 122,and the crosslink 108 is permitted.

The attachment mechanism 106 is locked down in a taper lock mechanism byactuating, or turning the nut 130. As the nut is turned and its threadsengage the threaded post 146, the conical expander 126 is urged “upward”through the nut 130, while the outer ring 162 of the split clamp 110 isurged “downward” toward the inner ring 160. As the conical expander 126moves, the flanges 150, 152 push against the lobes 140, 142 of the splitshell 128, and in turn the lobes expand and push against the interiorwall 114 of the interior cavity 112. Simultaneously, the flanges 150,152 are compressed around the inferior strut 104. Similarly, the collar164 of the split clamp 110 is compressed around the crosslink rod 108 asthe inner 160 and outer 162 rings of the clamp are urged together. Thenut 130 may be actuated until the resulting internal compressionprevents any further motion, and the mechanism is locked down.

The inferior implant 100 may be delivered in an assembled, but notlocked down, configuration. The crosslink rod 108 may be included in theassembly, provided separately, or excluded. The inferior implant 100 maybe delivered in combination with a superior implant, in which a clip orother temporary fastener holds the inferior articular surface to asuperior articular surface of the superior implant.

Referring to FIG. 5, inferior strut 104 is shown coupled to fixationassembly 300, which may also be termed an attachment mechanism. Fixationassembly 300 is configured to be implanted in a pedicle of a vertebra,and to be coupled to inferior implant 100 or another implant. Thefixation assembly 300 is polyaxially adjustable, and comprises afixation member 302, a base member 304, a split sphere 306, and a topnut 308. The inferior strut 104 is generally elongated in configuration,with a central portion 180, a first end or fixation portion which is aring 182, and a second end which is a strut post 184. The ring 180 maybe set at an angle relative to the central portion 180 and the strutpost 184. Conversely, the strut post 184 may be at an angle relative tothe central portion and the ring; also the central portion 180 may bestraight, bent or curved.

FIG. 6 is an exploded view of the inferior strut 104 and the fixationassembly 300. The fixation member 302, which may be a pedicle screw, hasa distal threaded bone-engaging portion 310, a shaft 312, and a proximalthreaded attachment portion 314. The base member 304 is cannulatedthroughout, and has a bone-engaging portion 316, a flange 318 and atapered portion 320. The bone-engaging portion may be tapered to providecompression to the surrounding bone, and may have a plurality of fins317 which prevent rotation of the base 304 in the bone. In alternateembodiments of the invention, the bone-engaging portion 316 may includeteeth, studs, posts, fins, or combinations thereof, or otheranti-rotation features, or no anti-rotation features. The taperedportion 320 may serve as an attachment portion, configured forattachment of an implant. At an open end of the tapered portion 320, atool engagement rim 322 includes a plurality of notches 324. Otherembodiments of the base may include threads or other features instead ofnotches configured to engage a tool. The split sphere 306 is sized tofit over the tapered portion 320 of the base 304, and includes aplurality of slits 328 which allow the sphere to be expandable. Thesplit sphere 306 may also include a tapered inner wall. The top nut 308has a threaded bore 332 and a flange 334 which encircles the nut 308.

The fixation assembly 300 may be delivered in a partially assembledstate or be assembled from the components described above. Duringimplantation, the fixation member 302 may be implanted in the pedicle ofthe vertebra using methods known in the art. The base member 304 is fitover the shaft of the fixation member 302. The split sphere 306 fitsover the tapered portion 320 of the base 304. The fixation portion, orring 182 of the inferior strut 104 is placed so it encircles the splitsphere 306, attaching the inferior strut to the fixation member.Optionally, split sphere 306 may be provided already captured in thefixation portion of the strut. Before or after placement on the base304, the ring 182 may be polyaxially adjusted around the split sphere sothat the inferior strut 104 attains a desired orientation. To lock downthe desired orientation, a compression lockout tool (not shown) engagesthe notches 324 of the tool engagement rim 322 on the base 304. Otherembodiments of the base may include a threaded tool engagementinterface, configured to engage with a threaded lockout tool. Thelockout tool provides compression on the split sphere 306, urging itfarther onto the tapered portion 320 toward the flange 318. As the splitsphere 306 moves down the tapered portion 320, it expands and engagesthe ring 182 of the inferior strut 104. Once all motion between thetapered portion 320, split sphere 306 and ring 182 is locked out, thetool is removed. The top nut is threaded onto the threaded attachmentportion 314 of the fixation member 302, to retain the base 304, sphere306 and ring 182 on the fixation member, and to further secure thebone-engaging portion 316 in the vertebra. Optionally, the base 304,split sphere 306, and ring 182 may be assembled and locked outindependently of the fixation member 302, then dropped onto the fixationmember 302 and retained with the top nut 308. The inferior implant 100may be secured to the inferior strut 104 before or after the inferiorstrut 104 is locked into position with the base 304 and split sphere306.

Referring to FIG. 7, the superior implant 200 is shown secured to thefixation assembly 300. The superior implant 200 may be monolithic andincludes a superior articulation surface 202 shaped to replace a naturalsuperior articular surface of a vertebra, a fixation portion or ring204, and may include at least one notch-like gripping feature 206. Thesuperior implant 200 may be secured to the fixation assembly 300 in thesame method as described previously for the inferior strut 104. The ring204 of the superior implant 200 is locked in position relative to thesplit sphere 306 and the base member 304. The base 304, split sphere 306and implant 200 may be dropped over an implanted fixation member 302,and the top nut 308 secured on the fixation member to retain theassembly. The superior implant 200 may be delivered in combination withan inferior implant 100, and the superior articular surface 202 may betemporarily clipped to the inferior articular surface 122.

Returning to FIG. 1, the components comprising the fixation assembly300, superior 200, 201 and inferior 100, 101 implants and crosslink 108may be implanted as follows. The pedicles are prepared for implantation,which may include removal of natural facet surfaces and bonepreparation, and may include a broaching step to shape the pedicles toreceive the base components. Broaching may ensure bone ingrowth andbetter mechanical retention of the bases and therefore the full implantsystem. Initially the fixation member 302 for each fixation assembly 300is driven into the pedicles to a prescribed or desired depth. A basemember 304 is placed on each fixation assembly 300, and thebone-engaging portion may be urged into the bone by pressing, tapping orother means. A split sphere 306 is placed on the bases in the caudalvertebra 2 intended for the superior implants, and the fixation portionsof the superior implants 200, 201 are placed over the split spheres, andlocked down relative to the fixation assembly as described previously.Alternatively, the split sphere 306 may be captured in the ring 204 ofthe implant 200 or 201, and the implant/ring assembly placed on the base304.

Next, the inferior implants 100, 101 are each assembled with an inferiorstrut 104, but not yet locked to the strut. A split sphere 306 iscaptured in the fixation ring 182 of each strut 104, and each inferiorimplant/strut/sphere assembly is placed on the attachment portion of thebase member 304 on a fixation member 302 on the cephalad vertebra 4. Anoffset distance between the inferior articular surface and the fixationassembly may be adjusted by moving the conical expander 126 relative tothe inferior strut 104. At this point, the inferior articular surfacesare aligned with the superior articular surfaces, and may be temporarilyclipped together to maintain the alignment. Additionally, theorientation of the inferior articular surface 122 may be polyaxiallyadjusted relative to the strut 104 by moving the split shell 128relative to the cavity 112. The inferior implant/strut assemblies arelocked down to the fixation assemblies.

The crosslink 108 may now be inserted through the collar 164 of thesplit clamp 110 of one inferior implant 100 or 101 and optionallythrough a prepared spinous process, and through the other collar 164 onthe remaining inferior implant 100 or 101. It is appreciated that as thecrosslink 108 is inserted, the split clamp 110 is rotatable about theclamp axis 111. Therefore, the crosslink 108 may be positioned to passthrough a spinous process, or may pass through soft tissue caudal to thespinous process. Alternatively, the crosslink 108 may be inserted beforethe inferior implants are locked down to the fixation assemblies. Theattachment assemblies 106 of each inferior implant 100, 101 are actuatedto lock down the implants, fixing the positions of the articularsurfaces 122, the inferior struts 104 and the crosslink 108 relative totheir respective fixation assemblies. Post-operatively, the articularsurfaces will be capable to articulate against one another, allowing alevel of natural spinal motion.

Some variation in the steps described above may occur. For example, theinferior articular body 102 may be available packaged with the superiorimplant 200, temporarily clipped together such that the articularsurfaces 122, 202 are in a desired alignment. In this instance, theinferior articular body 102 is inserted with the superior implant 200 asthe superior implant 200 is placed and locked with the fixation assembly300. Then the inferior strut 104 and the remaining components of theinferior implant 100, including the conical expander, split shell, andsplit clamp are assembled with the inferior articular body 102. Thefixation portion, or ring 182 of the inferior strut 104 is assembled andlocked down with the inferior fixation assembly 300. The insertion ofthe crosslink 108 and final lockdown is as described previously, and theclip is removed.

Alternatively, the inferior implant 100 may be available secured to aclip. The implant 100, with the attached clip, may be inserted adjacentto an already implanted and locked down superior implant, and theinferior and superior implants temporarily clipped together. Theinferior strut is adjusted and locked down to its fixation assembly. Theinsertion of the crosslink 108 and final lockdown of the inferiorimplant is as described previously, and the clip is removed.

System 10, and other facet replacement components disclosed herein, mayalso be implanted on multiple vertebral levels to provide facet jointreplacement across several levels. In a multi-level application,additional superior implants could be added to the fixation assemblies300 which secure the inferior struts 104, to extend the system in acephalad direction. Similarly, to extend the system caudally, additionalinferior struts coupled to inferior implants could be added to thefixation assemblies 300 which secure the original superior implants 200.Also, fusion rods (not shown) may be secured between fixation assemblies300 on adjacent vertebra to provide rigid fusion at a desired vertebrallevel.

FIG. 8 presents an alternative embodiment of a polyaxially adjustablefixation assembly 350 with an alternative embodiment of a superiorimplant 210. FIG. 9 presents an exploded view of fixation assembly 350,and FIG. 10 presents a cross-sectional post-assembly view of theassembly. With reference to all three figures, fixation assembly 350comprises a fixation member 352, a base member 354, a flanged splitsphere 356, a capture nut 358, and a top nut 360. The cannulated basemember 354 has a bone-engaging portion 362 which may includeanti-rotation features such as fins, teeth or studs. A tapered portion364 has a threaded lumen 366. The split sphere 356 includes a splitflange 368 which encircles one open end of the sphere. The capture nut358 has a threaded outer surface 370, while the top nut 360 has athreaded inner surface 372. Fixation assembly 350 may also be termed anattachment mechanism. It is appreciated that fixation assembly 350 maybe substituted for fixation assembly 300 in any fixation proceduredisclosed or depicted herein, and vice versa. Also, a combination offixation assemblies 300 and 350 may be used in an implant system.

The fixation member 352 is initially implanted into the pedicle, and thebase member 354 is inserted over the fixation member 352 and seated inthe bone. The split sphere is placed over the tapered portion 364 of thebase member 354. A fixation portion, or ring 212 of the superior implant210 is placed around the split sphere 356. At this point, the ring 212may be polyaxially adjusted to attain a desired orientation of thesuperior implant 210. To lock the orientation and position of thesuperior implant 200, a lockout tool (not shown) is actuated to effectthe taper lock. The lockout tool has an externally threaded inner shafttip which is engaged in the threaded lumen 366 of the base member 354.The lockout tool is actuated, using tensile force to simultaneously pullon the base member 354 with the inner shaft, and push on the flange 368of the split sphere 356 with an outer shaft. This force moves the splitsphere 356 farther onto the tapered portion 364. The split sphere 356expands and engages the ring 212 of the superior implant 210 until allmotion ceases and the position of the ring 212 is locked down. Thelockout tool is unthreaded and removed, and the capture nut 358 isthreaded into the tapered lumen 366, also capturing the flange 368 ofthe split sphere 356. The capture nut 358 is included to ensure thelong-term integrity of the lock. The top nut 360 is threaded onto thefixation member 352, and assists in holding the tapered base 362 againstthe bone surface. The top nut 360 and capture nut 358 may use the samedriver.

Referring to FIG. 11, a perspective posterior view depicts analternative embodiment of a bi-lateral facet joint replacement system20, implanted in two vertebrae. On the left lateral side, superior facetjoint prosthesis 22 comprises a superior implant 210 and a fixationassembly 300, secured to the first vertebra 2. The superior articularsurface articulates with an inferior articular surface of an inferiorfacet joint prosthesis 21, which comprises implant 400 and fixationassembly 500. An polyaxially adjustable attachment mechanism couples aninferior implant body to one end of an inferior strut 404, and acrosslink rod 109 which crosses a sagittal plane of the vertebrae. Anopposite end of the inferior strut is secured to the second vertebra 4by the fixation assembly 500. On the right lateral side, a mirror-imageof the system is implanted, including superior implant 211, secondfixation assembly 300, inferior implant 401, inferior strut 405 andfixation assembly 501. The crosslink rod 109 links the left inferiorimplant 400 to the right inferior implant 401. As previously set forth,only one lateral side of the system will be depicted and described.

FIG. 12 depicts the inferior implant 400, which comprises an inferiorarticular body 402, an inferior strut 404, and an attachment mechanism406 which polyaxially adjustably secures the articular body to theinferior strut. The crosslink rod 109 may be also secured to theinferior implant 400 by the attachment mechanism 406. Attachmentmechanism 406 may have two configurations: an adjustable configurationin which there is relative rotation between the inferior articular body402, the inferior strut 404 and the crosslink rod 109, and a lockedconfiguration in which the inferior articular body 402, the inferiorstrut 404 and the crosslink rod 109 are rigidly secured to each other.

FIG. 13 is an exploded view of the inferior articular body 402, inferiorstrut 404, crosslink rod 109 and the attachment mechanism 406. Theinferior articular body 402 is monolithic and comprises an inferiorarticulation surface 403 shaped to replace a natural inferior articularsurface of a vertebra, and a connection feature which has a roundedsurface 408, which in this embodiment is a spherical surface. Acompressible member 410 includes a conical portion 412 and a threadedpost 414. The conical portion 412 has an interior cavity 416 encircledby a plurality of expandable fingers 418. The interior cavity 416 isshaped to receive the rounded surface 408.

The inferior strut 404 has a first end 420 which is shaped as a rod andserves as the fixation portion for the strut. Other embodiments of theinferior strut may have a first end shaped as a ring or another shape. Asecond end 422 is shaped as a ring, and comprises a split ring clamp424, the split ring clamp having an inner ring 426, an outer ring 428,and a collar 430, which connects the inner and outer rings. The collar430 is oriented generally orthogonal to the inner and outer rings. Thecollar 430 is shaped to receive a split sphere 432, which has aninterior shaped to receive the crosslink rod 109. A nut 440 isconfigured to be threaded on the threaded post 414. Inferior strut 404may be straight, or it may be curved or bent such that the first andsecond ends 420, 422 are oriented at an angle relative to one another,as seen in FIG. 11.

FIG. 14 is a partial cross-sectional view of the attachment mechanism406 components in the locked configuration (the collar 430, split sphere432 and crosslink 109 are not visible in this figure). A clamp axis 411extends longitudinally through the attachment mechanism. As describedpreviously, the rounded surface 408 is received in the cavity 416 of thecompressible member 410. The split ring clamp 424 fits around thecompressible member 410, with the inner ring 426 around the conicalportion 412 and the outer ring 428 around the threaded post 414. Thecollar 430 fits around the split sphere 432, which receives thecrosslink rod 109. Also with reference to FIG. 13, when thus assembledbut not locked down, the attachment mechanism 406 is adjustable inmultiple ways. The inferior articular surface 403 may be polyaxiallyrotated relative to the inferior strut 404 and the crosslink rod 409 byrotation of the rounded surface 408. The split sphere encompassing thecrosslink rod 109 may be polyaxially rotated within the split ring clamp424 relative to the inferior strut 404 and the inferior articularsurface 403. A length of the crosslink rod 109 which extends through theattachment mechanism 406 may be adjusted. The inferior strut 404 hasrelative angular freedom of motion about the clamp axis 411. Theseadjustments provide relative rotation between the inferior articulationsurface 403 and the inferior strut 404 about three orthogonal axes. Inaddition, prior to lockdown, relative translation between the inferiorstrut 404, the inferior articulation surface 403, and the crosslink 109is permitted. An attachment mechanism 407, for the right side of thevertebrae, is configured as a mirror image of attachment mechanism 406.

The attachment mechanism 406 is locked down by actuating, or turning thenut 440. Lockdown of the attachment mechanism locks out both theposition of the inferior strut relative to the inferior articulationsurface, and the position of the crosslink. As the nut is turned and itsthreads engage the threaded post 414, the compressible member 410 isurged “upward” through the nut 440, while the outer ring 428 of thesplit ring clamp 424 is urged “downward” toward the inner ring 426. Asthe compressible member 410 moves, the tapered outer wall of the conicalportion 412 engages the inner surface of the inner ring 426.Simultaneously, the interior wall of the conical portion 412 exertscompressive force against the rounded surface 408 in the interior cavity416. Similarly, the collar 430 of the split ring clamp 424 is compressedaround the split sphere 432, which compresses around the crosslink rod109, as the inner 426 and outer 428 rings of the clamp are urgedtogether. The nut 440 may be actuated until the resulting internalcompression prevents any further motion, and the mechanism is lockeddown.

FIG. 15 is a perspective view of the fixation assembly 500, coupled toinferior strut 404. Fixation assembly 500 comprises a fixation member502, a base member 504, a top nut 506, a split ring clamp 508, a splitsphere 510 and a set screw 512. Fixation assembly 500 may also be termedan attachment mechanism, and it is adjustable, permitting polyaxialrotation of the inferior strut relative to the fixation member 502.

FIG. 16 is an exploded perspective view of the fixation assembly 500 andthe inferior strut 404. The fixation member 502 comprises a threadedbone-engaging portion 514 and a threaded attachment portion 516. Thebase member 504 comprises a receptacle 518 with a fixation bore 520sized to receive the fixation member 502, and a bone-facing side 519. Onthe bone-facing side 519 may be fins, pegs, teeth or other anti-rotationfeatures. The base member 504 may be dish-shaped as in FIG. 16, or maybe spherical, tapered, or another shape. Coupled to the receptacle 518is a tapered pedestal 521 which encircles a threaded attachment bore 522sized to receive the set screw 512. The top nut 506 is sized to fit intothe receptacle 518, and to be threaded onto the attachment portion 516of the fixation member. The split ring clamp 508 comprises an inner ring526, an outer ring 528, and a collar 530 which connects the inner andouter rings. An inner wall 527 of the inner ring 526 may be tapered. Theset screw 512 is threaded and sized to be received in the attachmentbore 522. The split sphere 510 is sized to fit around the rod-likefixation portion or first end 420 of the inferior strut 404, and sizedto fit inside the collar 530 of the split ring clamp 508. A mirror-imagefixation assembly 501 is configured to be implanted on the right side ofthe vertebra.

Returning to FIG. 15, fixation assembly 500 may be assembled and lockeddown as follows. Fixation member 502 is driven into a prepared pedicleat a desired depth. Base member 504 is placed on the fixation member 502so that the threaded attachment portion 516 fits through the fixationbore 520. The outer surface of the base member 504 may rest on theprepared pedicle. The top nut is threaded onto the attachment portion516 and actuated to secure the base 504 to the pedicle. The split sphere510 is captured in the collar 530 of the split ring clamp 508, and thefixation portion or rod portion 420 of the inferior strut may be slidinto the split sphere. The split ring clamp 508, now connected to theinferior strut 404, is placed on the pedestal 521 so that the inner ring526 surrounds the tapered pedestal 521. The set screw 512 is fit throughthe outer and inner rings 526, 528 and threaded into the attachment bore522. At this juncture the angle of the inferior strut 404 relative to aclamp axis 532, which may be parallel to the fixation member 502, may beadjusted. Also, the split sphere 510 may be polyaxially rotated withinthe collar 530, permitting polyaxial adjustment of the inferior strut404. When the preferred orientation of the inferior strut 404 relativeto the clamp axis 532, and the preferred orientation of the inferiorstrut to the collar 530 are reached, the fixation assembly 500 is lockeddown by actuating the set screw 512. As set screw 512 is tightened,outer ring 528 is urged toward inner ring 526. As the rings 526, 528come together, collar 530 is compressed around split sphere 510, whichin turn compresses around rod portion 420, locking its position. As setscrew 512 is turned, the tapered inner wall 527 of inner ring 526 isrigidly secured against the tapered pedestal 521, fixing the position ofthe split clamp ring 508 relative to the clamp axis 532.

With reference to FIGS. 11-16, the components comprising the fixationassemblies 300, 500, 501, superior 210, 211 and inferior 400, 401implants and crosslink 109 may be implanted as follows. The pedicles areprepared for implantation, which may include resection of natural facetsurfaces and bone preparation, and may include a broaching step to shapethe pedicles to receive the base components. Broaching may ensure boneingrowth and better mechanical retention of the bases and therefore thefull implant system. The fixation member 302 for each fixation assembly300 is driven into the pedicles of the caudal vertebra 2 to a prescribedor desired depth. A tapered base 304 is placed on each fixation member302. A split sphere 306 and superior implant 210, 211 is placed on thetapered bases 304 intended for the superior implants, and the taper lockis locked down relative to the fixation assembly as described previouslywith reference to FIGS. 8-10.

Before or after the fixation assemblies 300 are prepared, the fixationmembers 502 for the fixation assemblies 500, 501 are driven to a desireddepth in the cephalad vertebra 4. On the left side, base member 504 isplaced over the fixation member 502 and secured by the top nut 506. Theinferior strut 404 is assembled with the inferior articular body 402,and the attachment mechanism 406 as set forth previously, but not lockeddown. The split ring clamp 508 is assembled with the split sphere 510,and together they are slid onto the fixation portion of inferior strut404. The split ring clamp 508, now attached to the inferior strut 404and the inferior implant 400, is placed on the tapered pedestal 521 ofthe base member 504. On the right side, mirror-image duplicates of theleft components are similarly assembled. The inferior implants 400, 401are positioned so that the inferior articular surfaces are aligned withthe superior articular surfaces of the superior implants 210, 211, andthe inferior and superior articular bodies on each side may betemporarily clipped together to maintain the alignment. The inferiorimplant/strut assemblies are locked down to the fixation assemblies byactuating the set screws 512.

The crosslink 109 may now be inserted through the collar 530 of thesplit clamp 508 of one inferior implant 400 or 401 and through aprepared spinous process, and through the other collar 530 on theremaining inferior implant 400 or 401. Alternatively, the crosslink 109may be inserted before the inferior implants are locked down to thefixation assemblies. The attachment mechanisms 406 of each inferiorimplant 400, 401 are actuated to lock down the implants, fixing thepositions of the articular surfaces 403, the inferior struts 404 and thecrosslink 109.

Some variation in the steps described above may occur. For example, asseen in FIG. 17, an inferior articular body 470 may be availablepre-packaged temporarily locked to a coupler, or clip 550 with a plug570, which will be described in further detail below. Alternatively, agripping tool (not shown) may be used to hold the inferior articularbody 470. The attachment mechanism 406 and the inferior strut 404 (notseen) are assembled to the inferior articular body 470. The superiorimplant 210 is placed on and taper locked with the fixation assembly300, which is implanted in the pedicle. Using the clip 550 or grippingtool as a handle, the inferior implant articular body 470 with attachedstrut is placed adjacent to implanted superior implant 210 such thatposts on the clip 550 engage in openings on the superior implant, andthe inferior and superior articulation surfaces are aligned. Then thefixation portion of inferior strut 404 is slid into the split sphere 510and the split ring clamp 508 of the fixation assembly 500.(Alternatively, the split sphere and split ring clamp 530 may beassembled to the inferior strut 404 before it is placed adjacent to thesuperior implant). Polyaxiality of the split sphere 510 relative to thecollar 530 may be adjusted, and the set screw 512 is inserted and thefixation assembly 500 is locked down. The insertion of the crosslink 109and final adjustment and lockdown of attachment mechanism 406 is asdescribed previously. The clip 550 is unlocked and removed, allowingarticulation between the inferior and superior implants along theirrespective articular surfaces.

FIGS. 18A and 18B depict different perspective views of an alternateinferior strut 450. Inferior strut comprises a first end 452 and asecond end 454. Fixation portion or first end 452 is post-like, and maybe configured to be secured by a fixation assembly such as fixationassembly 500 seen in FIG. 15. Of course, other embodiments of the strutmay include a first end which is a ring or a different shape. The secondend 454 comprises a split ring clamp 456, which includes an inner ring458 and an outer ring 460, which are joined by a collar portion 462. Asseen in FIGS. 18A and 18B, the collar portion may be substantiallyorthogonal relative to the rings 458, 460, or it may be at anotherangle. Additionally, the angle of the second end 454 relative to thefirst end 452 may vary. Inferior strut 450 may be secured to anarticular body by an attachment mechanism in the same manner asdescribed for inferior strut 404; that is, a single actuating member maybe actuated to urge the inner and outer rings 458, 460 together andcompress the collar 462. Inferior strut 450 may differ from inferiorstrut 404 in features such as the position and/or angle of the splitrings relative to the collar, and the angle of the second end comprisingthe split ring clamp relative to axis of the first post-like end, amongothers. It is appreciated that any inferior strut disclosed herein maybe available in a variety of lengths, sizes, angles, and split ringclamp configurations.

Another alternative inferior strut (not pictured) may include separatepolyaxially adjustable attachment mechanisms for a crosslink and aninferior articular body. Such an alternative strut may include a firstring positioned and shaped to receive a polyaxially adjustable crosslinkrod, while a second ring is positioned and shaped to receive apolyaxially adjustable connection to an inferior articular body. Eachring may have an independent lockout mechanism such as a nut or screw.

FIG. 19 is a perspective view of the clip 550 and the plug 570. Clip 550may be monolithic and comprises a clip body 552, a handle 554, and twopairs of posts which extend substantially orthogonally from the body: apair of superior posts 556 and a pair of inferior posts 558. Theinferior posts 558 are cannulated, each having a bore 560 which extendsthe length of the post, through a rigid portion 563 to a deformableflexible split end 562. Each split end 562 includes at least one slot564 which extends partially along the length of the post 558, and aprotruding flange 566. The inferior posts 558 are shaped to receive aninferior facet joint implant, and the superior posts 556 are shaped toreceive a superior facet joint implant.

The plug 570 comprises a handle 571 and two wires 572 which are sized toextend through the bores 560 of the inferior posts 558 of the clip 550.When the plug 570 is inserted fully into the inferior posts 558, thewires 572 urge apart the flexible split ends 562 from a narrow firstconfiguration to an expanded second configuration in which the slots 564are widened, and the flanges 566 on each post are farther apart. Whenthe plug 570 is removed, the split ends 562 deform, moving toward oneanother from the expanded second configuration to the narrow firstconfiguration.

Returning to FIG. 17, the inferior articular body 470 is shown coupledwith the clip 550 and the plug 570. The inferior posts 558 extendthrough tubes 472 formed on the inferior implant 470, such that thesplit ends 562 and flanges 566 emerge outside of the tubes. The plug 570is fully inserted through the clip bores 560, and therefore the wires574 keep the split ends in the expanded second configuration. In theexpanded second configuration, the widened flanges 566 cause thediameter of the split ends 562 to be greater than the diameter of thetubes 472, retaining the articular body 470 and preventing the clip 570from being withdrawn from the inferior articular body 470. Thus lockedto the clip 550, the inferior articular body 470, with or without otherattached components such as a compressible member and/or an inferiorstrut, may be clipped to a superior implant.

Referring to FIG. 20, a perspective view shows the inferior articularbody 470 joined to the compressible member 410, attached to a clip 550.A direction arrow 580 indicates the direction in which the articularbody, compressible member and clip may be moved to align them with asuperior implant 211. The superior implant 211 is implanted in a pediclevia fixation assembly 350 previous to alignment with the inferiorarticular body 470, and a fixation assembly 500 is implanted into theadjacent pedicle. Using the handle 554, the clip may be moved until thesuperior posts 556 fit into openings 582 on the superior implant 211.Alternatively, as will be described below, clip 550 and inferiorarticular body 470 may be joined with strut 450 and with superiorimplant 211 into an assembly, and the assembly moved on to fixationmembers implanted in the pedicles.

As seen in FIG. 21, when the posts 556 are fully inserted into theopenings 582, inferior articulation surface 474 is aligned with superiorarticulation surface 584 in a preferred orientation. At this point, anappropriately sized and configured inferior strut may be chosen, itssecond end or split ring clamp coupled to the compressible member, andits first end or fixation portion coupled with and locked down to thefixation assembly 500. Additionally, a crosslink rod may be added andlocked down as the attachment mechanism is locked down. To unlock anddetach the clip 550, the plug 570 is removed, allowing the split ends562 to deform and return to the first narrow configuration and makingthem narrow enough to be withdrawn through the tubes 472. Then the clip550 may be removed.

Referring to FIG. 22, a multi-level facet joint replacement system 30 isshown implanted in a portion of a spine. Between adjacent vertebrae 8and 4, a first artificial facet joint replacement assembly replaces thenatural facet joints. The first assembly is linked to a secondartificial facet joint replacement assembly which replaces the naturalfacet joints between adjacent vertebrae 4 and 2. At the next level, thesecond artificial facet joint replacement assembly is linked to a fusionrod system which provides rigid fusion between vertebra 2 and the sacrum1. Crosslink rods connect the left lateral assemblies with the rightlateral assemblies.

Referring to FIG. 23, a lateral view shows the left lateral side ofsystem 30. System comprises many of the same components as system 20.Viewing the system in a cephalad to caudal direction, system 30 includesa fixation assembly 600 configured to be implanted in a first vertebra.A fixation portion of inferior strut 450 is secured by a split clamp tofixation assembly 600, and forms part of inferior facet implant 700.Inferior facet implant 700 articulates with a first superior facetimplant 800 which is secured to a first fixation assembly 300 which isconfigured to be implanted a second vertebra. An inferior strut 404 issecured by a split clamp to the first superior facet implant 800, andforms part of inferior facet implant 400. Inferior facet implant 400articulates with a second superior facet implant 810 which is secured toa second fixation assembly 300 which is configured to be implanted in athird vertebra. A fusion rod 900 is secured by a split clamp to thesecond fixation assembly 300, and extends to a fourth vertebra orsacrum, where it is configured to be secured by a fixation assembly 500.Two crosslinks 108 are coupled to the inferior implants and extendacross the sagittal plane to the right lateral side of the spine wherethey may be secured to right lateral side assemblies (as seen in FIG.22). Multi-level applications of this system are not restricted to threeor four levels; additional vertebral levels could be included by addingadditional components including inferior implants, superior implants,crosslinks, and/or fusion rods. It is appreciated that the sizes andconfigurations of components included in system 30 may vary to fitvarious vertebral sizes, offset distances and configurations andparticular patient anatomy. System 30 is polyaxially adjustable at eachvertebral level.

Referring to FIG. 24, a perspective view of fixation assembly 600,coupled with inferior strut 450 is shown. Fixation assembly 600comprises fixation member 602, base member 604 with tapered pedestal605, top nut 606, split ring clamp 608, split sphere 610 and set screw612. Fixation assembly 600 is similar to fixation assembly 500 seen inFIGS. 16 and 17, and is assembled similarly. Base member 604 may berotated about the axis of fixation member 602 prior to lockdown by topnut 606. Similarly, split ring clamp 608 may be rotated about the axisof the tapered pedestal 605 prior to lockdown by set screw 612. Basemember 604 may extend farther along the fixation member and deeper intothe bone than base member 504, and may include anti-rotation elementssuch as teeth, fins, and posts or studs, among others. It is appreciatedthat various component parts of the fixation assemblies herein disclosedmay be mixed and matched to form a variety of other alternatives. Forexample, base members 504 and 604 may be substituted for one another ifappropriate for the application, as may set screws 512 and 612.Similarly, fixation assembly 500 may be coupled with inferior strut 450,or another inferior strut or fusion rod, and fixation assembly 600 maycoupled with inferior strut 404, or another inferior strut or fusionrod.

Referring to FIG. 25, a perspective view of inferior facet implant 700is shown coupled to crosslink rod 108. Inferior facet implant 700comprises inferior articular body 402, inferior strut 450, andattachment mechanism 706 which couples the inferior articular body tothe strut. Attachment mechanism 706 comprises compressible member 410,split clamp 456, and nut 440. Inferior facet implant 700 may beimplanted in a multi-level application such as that seen in FIGS. 22 and23, or in conjunction with a superior facet implant, or singly.

Referring to FIG. 26A, a perspective view of a superior facet implant800 is shown, and in FIG. 26B, an alternative embodiment of a superiorfacet implant 810 is shown. Superior implant 800 comprises a superiorarticulating surface 802 and a fixation portion, or ring 804. Superiorarticulating surface 802 may be shaped to articulate with an inferiorfacet articulating surface. It is appreciated that the dimensions of thesurface 802 may vary, as can the orientation and angle of the surface802 relative to the remainder of the implant. The ring 804 is shaped toreceive a split sphere such as sphere 306 or 356, thus allowingpolyaxially adjustable coupling of the implant 800 to a fixationassembly such as fixation assembly 300. Adjacent the ring 804 is apedestal 806 which includes a bore 808. The pedestal 806 may be taperedand is configured to receive a split ring clamp, and the bore 808 isconfigured to receive a set screw, to form an attachment mechanismcapable of coupling an inferior strut, fusion rod or other rod-likemember to the superior implant 800.

As seen in FIG. 26B, superior facet implant 810 may be similar toimplant 800. Implant 810 comprises a superior articulating surface, afixation portion or ring 814, a pedestal 816 and a bore 818. The implantfurther comprises at least one notch 820 configured to receive a tool.Either superior implant 800 or 810 may include features to allow theimplant to be held in alignment with an inferior implant by a clip orgripping tool.

FIGS. 27-34 depict alternative embodiments of facet implant basemembers. Each base member comprises a tapered portion shaped to matewith an expandable member, or collet that is tapered inside andsubstantially spherical outside, such as split sphere 306, 356 or splitshell 128. The tapered surface facilitates a taper lock between the baseand the collet (and inferior or superior implant), thereby locking outadjustability between the implant and the base as described previouswith regard to FIGS. 6 and 9. Below the tapered portion may be a flangeto prevent subsidence and provide a stable surface against the adjacentbone, and to provide additional surface area for bone ingrowth. Inaddition, a generally cylindrical bone-engaging portion of the baseextends down into the pedicle of the vertebra. The bone-engagingportion, which may also be tapered forming a conical shape, may have anynumber of fins or other features (3-7 in preferred embodiments) whichmay project into the surrounding bone to resist rotation forces. Eachbase has a lumen extending throughout both the tapered portion andbone-engaging portion to fit over a pedicle screw or other fixationmember. The lumen may be cylindrical or may include a non-cylindricalindexing surface with one or more flat sections, shaped to receive ahexagonal driver or a driver of another shape, including triangular,square, pentagonal, or octagonal, among others. Additionally, each basemay have engagement features such as notches or threads which allow atool or gripping instrument to engage with and hold the base duringimplantation and lockout procedures. Implantation of each base mayfollow the same procedures as set forth previously for base 304. Baseswith fins or other protruding anti-rotation features may requireadditional bone preparation steps such as broaching to create slots inthe bone for the fins.

Each base member embodiment may differ in the number of fins thatradiate outward from the center axis to resist rotation. The length,width and taper of fins or other anti-rotation features may vary. Otherembodiments could use studs, pegs or posts instead of fins, or haveslots in the bone-engaging portion that extend downward into thepedicle. Also, the flange and/or bone-engaging portion may be coatedwith bone in-growth material such as porous material or hydroxyapatite,among others. Additional embodiments may incorporate sawteeth to allowfor self-guiding and/or self-cutting, therefore eliminating a separatepreparation step. It is appreciated that the bases disclosed herein maybe used with the fixation assemblies also disclosed herein, or in otherorthopedic applications employing bone-engaging fixation members forwhich the anti-rotation or other properties of the bases are desired.

The combination of a base member such as those disclosed herein with afixation member such as a pedicle screw may provide several advantagesto a pedicle screw alone. The contact area between the pedicle and thefixation assembly over which bending loads are distributed will beincreased, since the bone-engaging portion of each base provides agreater surface to bone contact area than a pedicle screw alone.According to Wolff's Law, a bone in a healthy person or animal willadapt to the loads it is placed under. If loading on a particular boneincreases, the bone will remodel itself over time to become stronger toresist that sort of loading. Increasing the bone contact area throughthe use of a base member may therefore result in strengthening of alarger portion of the bone around the implant fixation assembly.Additionally, less load may be placed on the pedicle screw, which mayresult in decreased likelihood of loosening of the screw over time.

FIG. 27A is a side view of a facet implant base member 850; FIG. 27B isan end view of the base 850; FIG. 27C is a perspective view of the base850; and FIG. 27D is cross-sectional view of the base 850. Base 850comprises a tapered portion 852 separated from a bone-engaging portion856 by a flange 854. A lumen 851 extends the length of the base, thelumen shaped to receive a fixation member such as 302, 352, 502, or apedicle screw, among others. A first end 858 includes several notches860 which are engagement features shaped to mate with a placement and/orlockout tool. In this embodiment, five evenly spaced fins 862 projectoutward from the bone-engaging portion 856. The fins 862 may preventrotation of the base 850 in the pedicle. A fillet 864 is located betweeneach fin and the adjacent fin and provides a transition between theflange 854 and the bone-engaging portion 856. In other embodiments ofthe base, there may be fewer or more fins, and the fins may be evenly orunevenly spaced, or paired, or grouped. The morphology of the fins mayvary; some fins may have sharp, well-defined edges while others may havemore rounded edges. The fins may taper between the flange and the distalend of the bone-engaging portion. Similarly, the sizes of the fillets864 may vary; a larger fillet will provide a less sharp, more continuoustransition between fins. Providing more gradual, less acute transitionsbetween features on the base may prevent the occurrence of low-loadareas where less bone in-growth might occur.

Referring to FIG. 28, an alternative embodiment of an implant basemember is shown. Implant base 870 has a tapered portion 872, a flange874 and a bone-engaging portion 876. A plurality of fins 878 extendoutward from the bone-engaging portion 876. The central lumen 871includes flat sections 873 interspersed with curved sections 875,allowing for engagement with a tool such as a pentagonal driver (notshown). The flat sections may provide a practitioner with immediateorientation of the fins 878 relative to the bone screw with which thebase is coupled, as well as to broached slots in the bone. The curvedsections 875 have a diameter outside the dimensions of the flatsections, allowing rotary bone preparation tools to be passed throughand used in the lumen. The tapered portion includes threads 877 whichmay extend throughout the tapered portion as shown or, in otherembodiments, may extend only partially through the tapered portion. Thethreads 877 are configured to engage with a placement and/or lockouttool, which may provide force to effect a taper lock between an implantand the base, as set forth previously.

Referring to FIG. 29, another alternative embodiment of an implant basemember is shown. Implant base 880 has a tapered portion 882, a flange884 and a bone-engaging portion 886. A plurality of fins 888 extendoutward from the bone-engaging portion 886. Each fin 888 is serratedwith several teeth 889, which may provide self-broaching of the boneduring implantation of the base.

Referring to FIG. 30, yet another alternative embodiment of an implantbase member is shown. Implant base 890 has a tapered portion 892, aflange 894 and a bone-engaging portion 896. A plurality of jagged fins898 extend outward from the bone-engaging portion 896. Each fin 898comprises a series of teeth 899 which may be graduated in size. Similarto implant base 880, the teeth may provide self-broaching duringimplantation, and may reduce the bone preparation needed prior toimplantation.

Referring to FIG. 31, another alternative embodiment of an implant basemember is shown. Implant base 900 comprises a tapered portion 902 and abone-engaging portion 904. A curved transitional area 906 connects thetapered portion and the bone-engaging portion. The transitional areaserves a similar function as the flange in other embodiments, preventingsubsidence of the implant. Two pegs 908, which may prevent rotation ofthe base, protrude outward from the bone-engaging portion 904.

Referring to FIG. 32, another alternative embodiment of an implant basemember is shown. Implant base 910 comprises a tapered portion 912 and abone-engaging portion 914. In this embodiment, the dish-shapedbone-engaging portion 914 has a greater diameter than the taperedportion 912. Bone-engaging portion 914 has a spherical bone-contactingsurface 915. The configuration of the bone-engaging portion 914 mayprevent subsidence of the implant, distribute the implant load over alarger surface area, and provide increased surface area for boneingrowth. A plurality of pegs 916 protrude from the bone-engagingportion 914 and may prevent rotation of the base. The pegs 916 arepositioned farther away from the central axis of the base 910, incomparison to the configuration of base 900 and pegs 908.

Referring to FIG. 33, another alternative embodiment of an implant basemember is shown. Implant base 920 comprises a tapered portion 922, aspherical transition portion 924 and a bone-engaging portion 926.Bone-engaging portion 926 is tapered and includes a plurality of holes928 which open into the central cannulated area, and may allowadditional bone ingrowth. Bone-engaging portion 926 may provide anarrower profile allowing for less disturbance of the pedicle duringpreparation and implantation.

Referring to FIG. 34, another alternative embodiment of an implant basemember is shown. Implant base 930 comprises a tapered portion 932, aspherical transition portion 934 and a tapered bone-engaging portion936.

FIG. 35 depicts a low profile facet replacement system 40 implanted onthe left side of two adjacent vertebrae 2, 4, and another low profilefacet replacement system 50 implanted on the right side. System 40comprises a superior facet implant 200 anchored to the pedicle byfixation assembly 300, and an inferior facet implant 1000 anchored by afixation assembly 1030. System 50 comprises a superior facet implant 201anchored by fixation assembly 300, and an inferior facet implant 1001anchored by a fixation assembly 1050. Systems 40 and 50 are mirrorimages of one another, with the exception that two different fixationassemblies, 1030 and 1050, are used to anchor the inferior implants. Inother embodiments of the invention, both systems 40 and 50 may includethe same fixation assemblies. A crosslink (not shown) may be coupled toeach assembly 40, 50 to provide a crosslink connection between theassemblies. The low profile design of the system results in an inferiorfacet joint implant that may have a reduced anterior-posterior dimensionwhen compared to other inferior facet joint implants.

Referring to FIG. 36A, a perspective view of inferior facet implant 1000is shown, and in FIG. 36B an alternative perspective view of the implantis shown. An implant articular surface is coupled with a low-profilecapture feature on the posterior side. The capture feature accepts astrut that has a spherical end. The spherical end mates with a concavityin the capture feature shaped to allow polyaxial range of motion. Alocking member, or set screw may be tightened down, applying compressionforces on the sphere, thereby locking it out. Specifically, inferiorimplant 1000 comprises an inferior articulation body 1002, an inferiorstrut 1004 and a set screw 1006. Inferior articulation body 1002comprises an inferior articulation surface 1008, a capture member 1010,and an attachment feature 1012. Attachment feature 1012 is shaped tomate with a crosslink clamp attachment (not shown).

Referring to FIG. 37, an exploded perspective view of implant 1000 isshown. Inferior articulation body 1002 may be monolithic, and includesthe inferior articulation surface 1008 which is shaped to replace anatural articular surface, and to articulate with a superiorarticulation surface. The capture member 1010 is coupled to body 1002and may be formed monolithically with the body 2002. The capture member1010 comprises a spherical concavity 1014 shaped to capture a sphericalend of the inferior strut 1004. Posterior to the spherical concavity1014 is a substantially circular concave threaded wall 1016 which isshaped to receive the set screw 1006. A generally posterior firstopening 1015 creates access to the concave threaded wall 1016 and thespherical concavity 1014. Adjacent to the spherical concavity 1014 andanterior to the concave threaded wall 1016 is a second opening 1017which allows for polyaxial adjustability of the inferior strut 1004.

The inferior strut 1004 comprises a strut body 1018 with a rod-likefixation portion or first end 1020 and a spherical second end 1022 whichhas a hemispherical surface 1023. The hemispherical surface isuninterrupted, meaning the surface is continuous across the hemisphereand there are no breaks or interruptions in the hemispherical surfacesuch as, for example, connection features extending outwardly from thehemispherical surface. However, the surface may be roughened tofacilitate engagement with the spherical concavity. A sphere diameter1022 d is less than a diameter 1015 d of the first opening 1015, butgreater than a diameter 1017 d of the second opening 1017, allowing thespherical second end 1022 to be captured in the spherical concavity1014. The strut body 1018 may further include a tapered portion 1024between the first and second ends. Features of the inferior strut 1004may vary, including but not limited to the size of the spherical secondend, and the degree of taper and placement of the tapered portion. Thefirst 1020 and second 1022 ends of the strut may be linearly orientedrelative to one another resulting in a radially symmetrical strut, orthey may be oriented at an angle. The set screw 1006 is exteriorlythreaded, and may include a drive feature 1026 (visible in FIG. 36A) ata first end 1028. At a second end 1029, the screw includes a sphericalpocket 1032 shaped to mate with the spherical second end 1022 of thestrut 1004.

Assembled as in FIGS. 36A and 36B, the spherical second end 1022 of thestrut 1004 fits into the spherical concavity 1014 of the capture member1010, and the first end 1020 of the strut extends out of the capturemember 1010. Prior to lockout, the second opening 1017 allows room forpolyaxial range of motion adjustment of the strut 1004 relative to thearticular surface 1008. The strut 1004 may be positioned with at least40 degrees of variability inside the capture member before lockout. Boththe second end 1022 and the spherical concavity 1014 may includeroughened surfaces to help facilitate engagement and lock-out betweenthe strut and the capture member. The set screw 1006 is threadiblyengaged in the threaded wall 1016, and the spherical pocket 1032 of theset screw mates with the spherical second end 1022 of the strut. Afteradjustment, the set screw 1006 is tightened down, applying compressionforces on the spherical second end, locking out all motion of the strut1004 relative to the articular surface 1008. When locked down, the setscrew 1006 may be entirely positioned within the capture member 1010,contributing to the low profile characteristics of the system.

System 40 may be implanted as follows, and it is understood that system50 may be implanted in a similar manner with a similar or differentfixation assembly in the cephalad vertebra 4. The pedicle of the caudalvertebra 2 is prepared for fixation member 302 and tapered base member304 which comprise fixation assembly 300. The pedicle of cephaladvertebra 4 is prepared for fixation assembly 1030, described in moredetail below. Existing natural facet surfaces may be resected asnecessary. Fixation assembly 300 is anchored in the prepared pedicle ofcaudal vertebra 2, and fixation assembly 1030 is anchored in thecephalad vertebra 4. Superior implant 200 is be placed and locked on tofixation assembly 300 so that the articular surface 202 is at aspecified facet angle. The spherical second end 1022 is placed in thecapture member 1010 of the inferior articular body 1002, and the setscrew 1006 may be engaged with the capture member 1010 but not tighteneddown. The fixation portion, or first end 1020 of inferior implant 1000is placed in the fixation assembly 1030 but not locked down. Inferiorimplant 1000 is polyaxially adjusted to align inferior articulationsurface 1008 with superior articulation surface 202, and locked down bytightening the set screw 1006. As seen in FIGS. 35 and 36, the capturemember 1010 is medially and posteriorly positioned relative to theinferior articular surface 1008. Additionally, the entire inferiorarticular surface 1008 may be positioned laterally of the sagittal plantof the caudal and cephalad vertebrae 2, 4. The fixation assembly 1030 islocked down. Alternatively, the fixation assembly 1030 may be lockeddown first, followed by the inferior implant 1000. Following lock-downof the fixation assemblies, the superior and inferior implants mayarticulate along their respective articular surfaces, preserving a levelof spinal motion.

In an alternative order of assembly, a fixation assembly 300 may beanchored in a prepared pedicle of caudal vertebra 2, and a fixationassembly 1030 anchored in a prepared pedicle of cephalad vertebra 4.Superior implant 200 and inferior articular body 1002 may be clippedtogether so their articulating surfaces are aligned, as describedpreviously, and dropped on to the fixation assembly 300. The sphericalsecond end 1022 of inferior implant 1000 is placed in the capture member1010 of the inferior articular body 1002, and the first end 1020 of theinferior implant 1000 is pivoted into the saddles of fixation member1030. Inferior implant 1000 is locked down by actuating set screw 1006,and fixation member 1030 is locked down by actuating its set screw.

Referring to FIG. 38, a perspective view of fixation assembly 1030 andinferior strut 1004 is shown. Fixation assembly 1030 comprises afixation member 1032, a capture member 1034, and a set screw 1036. Apair of saddles 1038 in the capture member 1034 is shaped to hold thefixation portion of strut 1004 or another rod-like member. Duringassembly, capture member 1034 may be rotated about the axis of thefixation member 1032. Prior to lockdown, the length of the strut 1004extending through the capture member 1034 may be adjusted. Lockdown isattained by turning the set screw 1036, thereby compressing the strut1004 within the saddles 1038.

Referring to FIG. 39, a perspective view of an inferior strut 1004captured in a fixation assembly is shown. Bone anchor assembly 1050comprises an eyelet screw body 1052, a compression sphere 1054, and aset screw 1056. The eyelet screw body 1052 is of monolithic, one-piececonstruction, although alternative embodiments could include separatescrew and eyelet pieces. Bone anchor assembly 1050 provides polyaxialand linear adjustments to allow for variations in pedicle to pedicleoffset dimensions.

FIG. 40 is an exploded view of bone anchor assembly 1050. Eyelet screwbody 1052 comprises a fixation portion or threaded bone-engaging portion1058, and an eyelet portion, or coupling member 1060. The couplingmember comprises a closed loop portion, through which a passageway 1062extends in an orthogonal orientation relative to the bone-engagingportion 1058. A concave wall 1064, shaped to substantially capture thecompression sphere 1054, encircles the passageway 1062. A countersink1066 flares out from the concave wall 1064 to the outer surface of thecoupling member 1060. The countersink 1066 provides increased surfacearea which may contribute to improved bone ingrowth. Posterior to, andcoincident with the center of the passageway 1062, is a threadedaperture 1068 encircled by a ring 1070. The threaded aperture 1068 maybe coaxial with the longitudinal axis of the threaded bone-engagingportion 1058, as in FIG. 40. On the exterior of the ring 1070 may be adrive feature 1072.

The compression sphere 1054 comprises a plurality of slots 1074interleaved with curved wall segments 1075. Multiple slots and wallsegments allow for local deformations, providing more points ofregistration against the concave wall 1064 when compressed and insertedinto the passageway 1062. The compression sphere 1054 has a compressiblebore shaped to receive an elongated member such as strut 1004. Thecompression sphere may have an uncompressed state, a first compressedstate in which it is compressed sufficiently to fit into the passageway1062 of the closed loop portion, with the outer diameter of the sphereequal to the diameter of the passageway. The sphere may further have asecond compressed state in which the slots 1074 and wall segments 1075are deformed about the strut 1004 sufficiently to both prevent movementof the strut and fix the position of the sphere relative to the couplingmember.

The set screw 1056 is of a twist-off configuration, in which a headsegment or drive element 1076 fractures from a threaded portion 1078 ata predetermined torque. The threaded portion 1078 has a spherical recess1080 which is shaped to mate with the compression sphere 1054. Theentire set screw 1056 may be cannulated, and the threaded portion 1078has an internal drive feature 1082 (visible in FIG. 39) which may be ahex drive feature.

FIG. 41 is a cross-sectional view of bone anchor assembly 1050 with aninferior strut 1004 locked in the compression sphere 1054. Duringimplantation, eyelet screw body 1058 is driven into a prepared pedicle,with compression sphere 1054 compressed to the first compressed stateand captured in the passageway 1062 of the closed loop portion. Thefixation portion of inferior strut such as strut 1004, or other strut orrod which the bone anchor assembly 1050 is anchored, is inserted throughthe compression sphere 1054. The strut and sphere may be polyaxiallyrotated to attain a desired orientation. The combination of therotatable sphere and flared countersink 1066 allows for a range ofmotion of +/−35 degrees, for a total included angle of 70 degrees. Also,the length of the strut extending through the sphere 1054 may beadjusted to attain a desired pedicle to pedicle offset. When the desiredposition and orientation of the strut are attained, the set screw 1056is threaded through the threaded aperture 1068 and torqued to lock outmovement between the sphere, strut and eyelet. The set screw 1056directly contacts the compression sphere 1054, the spherical recess 1080mates with the compression sphere 1054 and the sphere 1054 is compressedaround the strut 1004, and deformed within the closed loop portion,forming many areas of contact between the sphere 1054 and the concavewall 1064. At this second compressed state, movement of the strut isprevented and the sphere is locked in a fixed position relative to thecoupling member. At a predetermined torque, the drive element 1076fractures from the threaded portion 1078 of the set screw 1056.

FIG. 42 illustrates an alternative fixation assembly 1090, which may bedescribed as a split eyelet clamp bone anchor assembly. Fixationassembly 1090 comprises fixation member 1092, a split ring clamp 1094, acompression sphere 1096, and a set screw 1098. Fixation assembly 1090may be used to anchor an inferior facet joint implant such as implant1000 or implant 400, or another rod-like member such as a fusion rod, toa vertebra.

Referring to FIG. 43, an exploded view of fixation assembly 1090 isshown. Fixation member 1092 has a threaded portion 1100 and anattachment portion 1102 which may be tapered. The attachment portion1102 may include a drive feature 1104 such as a hex drive. The splitring clamp 1094 is of two piece construction, comprising a lower clampbody 1106 and an upper clamp body 1108. The lower clamp body 1106 iscannulated with a bore 1110 which may be tapered, in order to form atapered connection between fixation member 1092 and lower clamp bodyportion 1106. Lower clamp body 1106 further comprises an outer surface1112 which may be a bone ingrowth surface; a spherical pocket 1114shaped to receive the compression sphere 1096; a threaded lower ring1116 shaped to receive the set screw 1098; and a linking feature 1118which may be a groove shaped to mate with a corresponding feature on theupper clamp body 1108. Alternative embodiments of lower clamp body 1106could include anti-rotation features configured to engage withsurrounding bone to prevent rotation of the assembly, including but notlimited to fins, teeth, studs, and pins. The compression sphere 1096 isa C-shaped split sphere, and includes a plurality of slits 1120. Upperclamp body 1108 comprises a linking feature 1122 which may be aprotrusion, a spherical pocket 1124, and an upper ring 1126. The setscrew 1098 comprises a threaded portion 1128 and a head 1130.

Fixation member 1092, coupled with lower clamp body 1106, may beanchored in a prepared pedicle. A lockout tool may be implemented toeffect a taper lock between the fixation member and the lower clampbody. The compression sphere 1096 may be coupled with fixation portionof an inferior strut such as strut 1004, or another rod-like member,such that a desired length of the strut extends through the sphere so asto match a vertebral offset. The coupled sphere 1096 and strut areplaced in the spherical pocket, and the sphere may be rotated until thestrut is at a desired orientation. The upper clamp body 1108 is coupledto the lower clamp body 1106 such that the linking features 1118, 1122mate and the upper ring 1126 is aligned with the lower ring 1116. Theset screw 1098 is inserted through the upper ring 1126 and threaded intothe lower ring 1116. As the set screw is actuated, the engagement of thethreaded portion 1128 with the threaded lower ring 1116 draws the lowerring upward, and the head 1130 presses down on the upper ring 1126. Asthe rings 1116, 1126 are thus urged together, the upper and lower clampbodies 1106, 1108 compress around the compression sphere 1096, whichcompresses around the strut. Motion of the sphere 1096 and the strutrelative to one another and to the remainder of the assembly 1090 islocked out.

Referring to FIG. 44, an alternative method of securing inferior andsuperior facet joint implants using a coupling clip is illustrated in apartial exploded view. In such an implantation procedure, a fixationmember such as fixation member 302 is anchored in a prepared pedicle ofa caudal vertebra, and base member 304 is placed on the fixation member.In the adjacent cephalad vertebra, a fixation member such as 502 isimplanted in a prepared pedicle and a base such as 504 is coupled to thefixation member. Superior implant 211, split sphere 306, clip 550,inferior body 470, and compressible member 410 may be providedpre-assembled as assembly 555 in a sterile package. Clip 550 securessuperior implant 211 to inferior body 470 such that superiorarticulation surface 584 and inferior articulation surface 474 are in adesired orientation relative to one another. The clip body 552, combinedwith the rigid superior posts and rigid portions of the inferior posts,provides a rigid feature which holds the superior 584 and inferior 474articulation surfaces in a fixed alignment. Assembly 555 is placed overfixation member 302 so that sphere 306 fits onto tapered based 304.Superior implant 211, with attached inferior body 470, may bepolyaxially adjusted to a preferred orientation relative to fixationmember 302 and the caudal vertebra. When a desired orientation isattained, a compression tool may be used to effect a taper lock, asdescribed above with reference to FIG. 6, and set screw 308 is actuatedto lock down the assembly 555 to the fixation member 302.

A sphere 510 and split ring clamp 508 are placed on the first end 452,or fixation portion, of inferior strut 450 at a desired linear position.Inferior strut 450 is placed such that its second end 454 encirclescompressible member 410, and, generally simultaneously, the split ringclamp 508 on the first end 452 of the strut fits over the pedestal 521of base member 504. Compressible member 410 may be adjusted relative toinferior body 470, and sphere 510 may be polyaxially rotated to adjustinferior strut 450 relative to base member 504. Optionally, a crosslinksuch as 108 or 109 (not shown) may be placed in split ring clamp 456.The final position and orientation of the inferior strut 450 is lockedout by actuating set screw 512 and nut 130. Plug 570 is removed fromclip 550, allowing split ends 562 to deform and contract. Clip 550 iswithdrawn from inferior body 470 and superior implant 211, and removed.Once the clip is removed, the superior and inferior implants mayarticulate along their articular surfaces, allowing a level of naturalspinal motion.

Referring to FIG. 45A, an alternate embodiment of a coupling clip isshown. Clip 1200 is of one-piece construction, and is shaped to couplean inferior facet replacement implant such as implant 100 with asuperior facet replacement implant such as implant 210. Clip 1200 mayretain the implants such that the inferior and superior articulationsurfaces are held at a desired relative position. A portion of the clip1200 is deformable and may be flexed to detach the clip from at leastone of the implants.

Clip 1200 comprises a first end 1202 and a second end 1204, and the endsare linked by a connecting portion 1206. First end 1202 comprises arigid shoulder 1208, and at opposing ends of the rigid shoulder 1208 area tab 1210 and a post 1212. The tab 1210 and post 1212 are also rigid,and are shaped to couple with and align the inferior and superiorimplants. A recess 1220 is located on the shoulder 1208. Similarly,second end 1204 comprises a rigid shoulder 1214, tab 1216, post 1218,and recess 1222. Tabs 1210, 1216 are shaped to receive an inferior facetjoint implant, and posts 1212, 1218 are shaped to receive a superiorfacet joint implant. Connecting portion 1206 is deformable, and when itis flexed, first end 1202 rotates about the axis of post 1212, andsecond end 1204 rotates about the axis of post 1218, such that tabs1210, 1216 are urged apart.

FIG. 45B is a perspective view of clip 1200 coupled to an inferior facetjoint implant 1230. Inferior facet joint implant 1230 is similar toimplant 100 seen in FIG. 2, but includes an alternative inferiorarticular body 1232. Inferior facet joint implant 1230 comprisesinferior articular body 1232, conical expander 126, split shell 128 (notvisible), split clamp 110, top nut 130, inferior strut 104, and sphere356 which may be captured in the fixation portion or first end 182 ofthe inferior strut. Inferior articular body 1232 comprises an inferiorarticular surface 1234 and a set of slots 1236 which are shaped toreceive the tabs 1210, 1216 of the clip 1200. Inferior facet jointimplant 1230 may be delivered coupled to clip 1200. Packaging (notshown) may be shaped to prevent connecting portion 1206 from flexing,and to keep posts 1212, 1218 in a fixed position.

Referring to FIG. 46, clip 1200 and implant 1230 are shown gripped by adelivery tool 1300. Additionally, superior facet implant 1200 is showncoupled to fixation assembly 350. The delivery tool 1300 compriseshandles (not shown), a shaft 1302, a hook 1304 which may be actuated togrip and release the clip 1200, and a pair of pegs 1306, 1308. Uponremoval of the packaging described above, the delivery tool 1300 may beconnected to the clip 1200 via the hook 1304 which hooks on theconnection portion 1206, and the pegs 1306, 1308 which protrude into therecesses 1220, 1222. The spacing of the pegs keeps the posts 1212, 1218of the clip 1200 in a proper position for coupling with the superiorimplant 210. The hook 1304 may prevent premature flexure of theconnection portion 1206. The delivery tool 1300 may be manipulated toposition the clip 1200 and implant 1230 such that the posts 1212, 1218fit into the holes 216, 218 on the superior implant, thus properlyaligning the inferior 1234 and superior 214 articulation surfacesrelative to one another.

Referring to FIG. 47, a flexing tool 1320 is shown coupled to theconnecting portion 1206 of the clip 1200. Flexing tool 1320 is co-axial,and comprises handles (not seen), a shaft 1322, and two grippingfeatures 1324, 1326. The gripping features 1324, 1326 are shaped andpositioned to grip two locations on the connecting portion 1206. Theflexing tool 1320 may be activated to move the gripping features 1324,1326 relative to one another such that the connecting portion 1206 isflexed.

With reference to FIGS. 45-47, one method of implanting inferior facetreplacement implant 1230 and superior facet replacement implant 210 isas follows. It is understood that steps may occur in the orderpresented, or in a different sequence. It is further understood thatright and left facet joint replacements may be implanted during the sameprocedure and optionally linked via a crosslink. Fixation assembly 350is implanted into a prepared pedicle, and fixation portion or ring 212of superior implant 210 is positioned and taper-locked onto the fixationassembly, as described previously. A second fixation assembly 350 (notshown) is implanted into the pedicle of the adjacent cephalad vertebra,minus sphere 356, capture nut 358 and top nut 360. Clip 1200 andattached inferior implant 1230 are removed from sterile packaging andcoupled to delivery tool 1300. The delivery tool 1300 is manipulated toposition sphere 356 onto fixation assembly 350, and posts 1212, 1218 ofthe clip 1200 into the holes 216, 218 of the superior facet implant 210.As the clip is positioned, polyaxial adjustment may occur at severaljunctures, allowing adjustment of the inferior articular surface 1234relative to the fixation assembly 350. Sphere 356 may rotate relative tothe fixation assembly 350, the linear position of inferior strut 104 maybe adjusted to match the offset distance between the vertebrae, and thesplit shell 128 may rotate within the inferior articular body 1232. Whenthe clip 1200 is properly positioned so that the posts 1212, 1218 fitinto the holes 216, 218 and the articular surfaces 214, 1234 arealigned, the delivery tool 1300 may be triggered to release the clipfrom the hook 1304, and the delivery tool 1300 is removed. A crosslinksuch as 108 may be positioned in the split clamp 110. The fixationassembly 350 is taper-locked relative to the sphere 356 and inferiorstrut 104, and capture nut 358 and top nut 360 are added to secure theassembly. Nut 130 is actuated on conical expander 126 to lock down therelative orientation of inferior strut 104 and inferior articular body1232, and lock position of crosslink 108. Flexing tool 1320 is attachedto the connecting portion 1206 of the clip 1200, and activated to flexthe connecting portion. As the connecting portion 1206 of the clip 1200is flexed, shoulder 1208 rotates relative to the axis of post 1212, andshoulder 1214 rotates relative to the axis of post 1218, and tabs 1210,1216 are urged apart, and out of slots 1236. Thus, clip 1200 is detachedfrom inferior implant 1230 and also can be urged away from superiorimplant 210.

The coupling clips disclosed herein may be made in a variety of sizes,and with varied dimensions, to fit implants configured for differentvertebral levels. Other embodiments of clips may include differentdeformable retention features, different alignment features, and/ordifferent features shaped to receive the superior and inferior implants.Coupling clips without deformable features or plugs, and/or with otherattachment features are contemplated within the scope of the invention.In addition, trial clips in a variety of sizes and configurations may beprovided, to allow the practitioner to choose the correct size orconfiguration of implant. Trial clips may include integrated superiorand/or inferior implant trials. A trial clip and implant may be used toselect the proper length of inferior strut to match an offset distancebetween the vertebrae. Specifically, fixation members and base membersmay be secured in adjacent vertebrae, and a succession of trials, eachcomprising a clip retaining an inferior and optionally a superiorimplant may be positioned on the bases, until the proper length ofinferior strut is determined. Then the sterile package containing theproper choice of clip and implants may be opened and the appropriateclip and implants secured to the base members. Use of the trialsprevents practitioners from unnecessarily opening more than one sterilepackage of implants to determine a correct fit.

Referring to FIG. 48, a posterior perspective view shows an alternativeembodiment of a bi-lateral facet joint replacement system. System 60comprises superior facet joint implants 200, 201 each anchored in thecaudal vertebra 2 by a fixation assembly 300, and inferior facet jointimplants 1400, 1401 each anchored in the cephalad vertebra 4 by afixation assembly 1030. A crosslink 1450 links implants 1400, 1401.System 60 is configured so that a medial-lateral distance betweenimplants 1400, 1401 is adjustable by sliding the implants along the axisof the crosslink to vary the location of the implants relative to oneanother. The articular surfaces of the implants 1400, 1401 may also berotated about the axis of the crosslink and oriented in a polyaxialmanner with respect to all other components. Additionally, each inferiorstrut may be independently rotated relative to the crosslink and thearticulation surfaces, and medial-laterally adjusted relative to thecrosslink and the articulation surfaces. This high degree ofadjustability may allow practitioners to tailor the system to thespecific morphology of a patient's spine, including patients withextreme morphology.

FIG. 49 displays a caudal perspective view of implants 1400, 1401, andcrosslink 1450. Superior implants 200, 201 are included to show thealignment of the articulation surfaces of the implants. As with previousembodiments, implants on one lateral side will be described and it maybe assumed that the other lateral side is a mirror image, unlessotherwise specified. Of course, components on either side may vary insize and positioning. It is also noted that an alternative embodiment ofthe invention could include a system omitting the crosslink 1450.

Referring to FIG. 50, an exploded view of implants 1400, 1401 andcrosslink 1450 are shown. Inferior facet joint replacement implant 1400comprises an inferior articulation body 1402, a coupling body which maybe a tulip body 1404, compressible member 1406, inferior strut 1408, andset screw 1410. Inferior articular body 1402 comprises an inferiorarticulation surface 1412 and a spherical member 1414. The sphericalmember 1414 is polyaxially rotatable within the compressible member 1406and the tulip body 1404 prior to lockout, so that the inferiorarticulation surface 1412 may be aligned at a desired orientation. Thespherical member 1414 may or may not comprise a flattened section 1416for clearance.

The tulip body 1404 is generally U-shaped. A rounded cavity 1418, sizedand shaped to receive the compressible member 1406, is partiallyenclosed by a concave wall 1420. Two opposably oriented sidewalls 1422,1424 extend posteriorly from the concave wall. A portion of the interiorsurfaces of the sidewalls are threaded to receive the set screw 1410.Two opposably oriented saddles 1426, 1428 are formed posterior to theconcave wall 1420 and between the sidewalls 1422, 1424.

The compressible member 1406 comprises an interior cavity 1432 partiallyenclosed by a plurality of fingers 1434. A trough 1436 extends acrossthe compressible member 1406 posterior to the interior cavity 1432, andan opening 1438 may or may not connect the trough 1436 to the interiorcavity. The interior cavity 1432 is shaped to receive the sphericalmember 1414. The outer surface of the fingers 1434 are sized and shapedto deflect inward as the member is pressed in an anterior directionthrough the cavity 1418 of the tulip body 1404. Additionally, the outersurface of the compressible member is shaped such that the trough 1436maintains alignment with the saddles 1426, 1428 of the tulip body 1404.

The crosslink 1450 is shaped as a longitudinally split cylinder. Itcomprises a half-pipe body 1452 with a first end 1454 and a second end1456. The half-pipe body 1452 is sized and shaped to be received in thetrough 1436 of the compressible member 1406, and sized and shaped toreceive a portion of each inferior strut 1408.

The inferior strut 1408 comprises a fixation portion, or first end 1460and a second end 1462 connected by a transition portion 1461. The strut1408 is generally L-shaped with the first and second ends at approximateright angles relative to one another, although other embodiments couldinclude struts with angles of more or less than 90 degrees or strutsthat may be bent to the desired angle. The first end 1460 iscylindrical, sized and shaped to be received by fixation assembly 1030.The second end 1462 is sized and shaped to be received in the half-pipebody 1452.

FIG. 51 is a partial cross-sectional caudal view of inferior implants1400, 1401, superior implants 200, 201, and crosslink 1450. Referring toFIGS. 48 and 51, one method of implanting system 60 into a portion of aspine may be as follows. Fixation assemblies 300 and superior implants200, 201 are implanted in the caudal vertebra 2 and locked out, asdescribed previously. The pedicle screw and tulip portions of fixationassemblies 1030 are anchored in the cephalad vertebra 4. Implants 1400and 1401, minus their respective inferior struts, are positioned so thattheir inferior articular surfaces are aligned with the superiorarticular surfaces of implants 200 and 201 at a desired orientation andtemporarily held together. Positioning the implants 1400, 1401 maycomprise polyaxially adjusting the spherical members and attachedarticular surfaces, and/or translating the inferior articular surfacesalong the medial-lateral axis of the caudal 2 and cephalad 4 vertebrae.Alternatively, the inferior facet implants 1400, 1401 may be temporarilyattached to the superior facet implants 200, 201 in the desiredorientation allowing opposing articulating surfaces to be implantedtogether. The crosslink may be inserted from a posterior approach intothe tulip body 1404 of implant 1400 and maneuvered until the first end1454 of the crosslink is within the saddles 1426, 1428 and contactingthe trough 1436 of the compression member 1406. The crosslink may thenbe slid through the hole in the spinous process or interspinous processtissue until the second end 1456 is within the saddles of the tulip body1404 of implant 1401 and contacting the trough 1436 of the compressionmember 1406. Alternatively, the crosslink 1450 may be dropped down intoboth tulips at the same time if there is no bone or tissue in the way.The crosslink 1450 is then rotated about its longitudinal axis until therounded outer wall of the half-pipe body 1452 rests in the saddles ofboth tulip bodies and contacts the troughs 1436 of the compressionmembers 1406. In this position, the crosslink 1450 may not popposteriorly out of the tulip bodies, but may be slidably adjustablealong the medial-lateral axis.

With the crosslink 1450 spanning the tulip bodies 1404 as described, theinferior struts 1408 may be placed in the system, one on each lateralside. The left inferior strut is placed so that its second end 1462 isreceived in the first end 1454 of the crosslink 1450, and the rightinferior strut is placed so that its second end 1462 is received in thesecond end 1456 of the crosslink. Trial inferior struts in a variety ofsizes may be provided to aid in determining proper strut size. Once theproper size of inferior strut is chosen, each appropriately sized strutis placed in the crosslink and may be slidably adjusted along themedial-lateral axis of the crosslink and rotated about that axis. Eachstrut is rotated until its first end 1460 is received in the saddles1038 of the capture member 1034 of its respective fixation assembly1030. The struts may also be adjustable along the cephalad-caudal axisof the vertebrae. Until lockdown, the capture members 1034 may bepolyaxially rotated to desired positions to receive and adjust theinferior struts 1408.

Once the struts 1408 are placed and adjusted, the set screws 1410 areactuated in the tulip bodies 1404 to lock out motion of the sphericalmembers 1414, crosslink 1450, and struts 1408. As set screw 1410 istightened, its threads engage with the threaded inner walls of sidewalls1422, 1424. The tulip body 1404 is drawn posteriorly or “upward” and theset screw moves anteriorly or “downward”. This opposing motioncompresses together the first end 1462 of the inferior strut, thecrosslink 1450, and the compressible member 1406, locking out theirmotion. The fingers 1434 of the compressible member 1406 are urgedtogether by the concave wall 1420 of the tulip body, in turn compressingthe compressible member about the spherical member 1414, and locking outmotion of the inferior articular body 1402. Set screws 1036 are actuatedin the capture members 1034 to lock out motion in the fixationassemblies 1030. The inferior facet implants 1401 and 1402 are thenallowed to articulate against their respective superior facet implants200 and 201 by removing any temporary holding device.

Referring to FIG. 51, it is noted that the orientation of inferiorimplant 1400 relative to superior implant 200 is not the same as theorientation of inferior implant 1401 relative to superior implant 201,however there is enough adjustability to allow them to have similaralignments. It is appreciated that the medial-lateral and rotatableadjustability of the tulip bodies 1404 and the struts 1408, along withthe polyaxial adjustability of the inferior articular body 1402, allowfor precise yet differing orientation of the implants relative to oneanother. This adjustability, along with the adjustability in thefixation assemblies 1030 connecting the first end 1460 of the struts1408, allow the system 60 to be adjusted to a full range of vertebralmorphologies.

The present invention includes variances of the systems hereindescribed. Alternative embodiments may include different geometries andintermediate parts. Changes in the geometry, especially on the ends ofthe inferior strut, could be made to facilitate instrumentation oroverall function. It is appreciated that various features of theabove-described examples can be mixed and matched to form a variety ofother alternatives. For example, a strut, fusion rod or other rod-likemember may be anchored or locked down by any of the fixation assembliesherein disclosed. Applications of the present invention may includesingle- or multi-level facet joint replacement with motion preservation,or other iterations in which a rod or rod-like member is fixed to asecond member to attain spinal fusion.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. As such, thedescribed embodiments are to be considered in all respects only asillustrative and not restrictive. The scope of the invention is,therefore, indicated by the appended claims rather than by the foregoingdescription. All changes which come within the meaning and range ofequivalency of the claims are to be embraced within their scope.

1. A system for replacing at least a portion of a natural facet jointthat controls motion between a first vertebra and a second vertebra, thesystem comprising: a first fixation member implantable in a pedicle ofthe second vertebra; and an inferior facet joint implant comprising: aninferior articular surface shaped to replace a natural inferior facetarticular surface of the second vertebra; an inferior strut formedseparately from the inferior articular surface, the inferior strutcomprising a first end and a second end, wherein the first end issecurable to the first fixation member and the second end comprises asphere having an uninterrupted hemispherical surface; and an attachmentmechanism comprising a capture feature shaped to receive the sphere, theattachment mechanism further comprising an adjustable configuration inwhich the inferior articular surface is connected to the second end in amanner that permits relative rotation between the inferior articularsurface and the second end about three orthogonal axes, and a lockedconfiguration in which the inferior articular surface is rigidly securedto the second end.
 2. The system of claim 1, wherein the capture featureis formed monolithically with the inferior articular surface.
 3. Thesystem of claim 1, wherein the capture feature is medially positionedrelative to the inferior articular surface when the first end is securedto the first fixation member and the attachment mechanism is in thelocked configuration.
 4. The system of claim 1, wherein the attachmentmechanism further comprises a locking member, wherein the locking memberis actuatable to exert force against the sphere to restrict rotation ofthe sphere relative to the articular surface to provide the lockedconfiguration.
 5. The system of claim 4, wherein the capture featurecomprises a first opening, wherein the locking member is shaped to bereceived in the first opening such that the locking member is entirelypositioned within the capture feature when the attachment mechanism isin the locked configuration.
 6. The system of claim 5, wherein thecapture feature further comprises a second opening contiguous with thefirst opening, the first opening having a first diameter which isgreater than a sphere diameter of the sphere, the second opening havinga second diameter which is less than the sphere diameter, wherein thesphere is receivable through the first opening.
 7. The system of claim1, further comprising: a second fixation member implantable in the firstvertebra; and a superior facet joint implant securable to the secondfixation member, the superior facet joint implant comprising a superiorarticular surface shaped to replace a natural superior articular surfaceof the first vertebra.
 8. The system of claim 1, wherein the inferiorstrut further comprises a tapered portion positioned between the firstend and the second end.
 9. The system of claim 1, wherein the inferiorstrut is radially symmetrical such that a cross-section taken orthogonalto the longitudinal axis of the inferior strut at any location along theinferior strut is circular in shape.
 10. The system of claim 1, whereinthe entire inferior articular surface is positioned laterally of thesagittal plane of the first and second vertebrae when the attachmentmechanism is in the locked configuration.
 11. The system of claim 1,wherein the inferior facet joint implant is one of a left inferior facetjoint implant configured to be implanted on the left lateral side of thesecond vertebra to replace the natural left inferior facet joint, and aright inferior facet joint implant configured to be implanted on theright lateral side of the second vertebra to replace the natural rightinferior facet joint.
 12. The system of claim 1, wherein the firstfixation member comprises an adjustable configuration in which the firstend of the inferior strut is connected to the first fixation member in amanner that permits relative rotation between the first end and thefirst fixation member about three orthogonal axes, and a lockedconfiguration in which the first end is rigidly secured to the firstfixation member.
 13. A system for replacing at least a portion of anatural facet joint that controls motion between a first vertebra and asecond vertebra, the system comprising: a first fixation memberimplantable in a pedicle of the second vertebra; and an inferior facetjoint implant comprising: an inferior articular surface shaped toreplace a natural inferior articular surface of the second vertebra; aninferior strut formed separately from the inferior articular surface,the inferior strut comprising a first end and a second end, wherein thefirst end is securable to the fixation member; and an attachmentmechanism comprising a capture feature shaped to receive the second end,the capture feature comprising a first opening having a first diametergreater than a diameter of the second end to allow passage of the secondend through the first opening, and a second opening contiguous with thefirst opening, the second opening having a second diameter less than thediameter of the second end to block passage of the second end throughthe second opening.
 14. The system of claim 13, wherein the capturefeature is formed monolithically with the inferior articular surface.15. The system of claim 13, wherein the second end comprises a spherewith an uninterrupted hemispherical surface.
 16. The system of claim 13,wherein the attachment feature comprises an adjustable configuration inwhich the inferior articular surface is connected to the second end in amanner that permits relative rotation between the inferior articularsurface and the second end about three orthogonal axes, and a lockedconfiguration in which the inferior articular surface is rigidly securedto the second end.
 17. The system of claim 16, wherein the capturefeature is medially positioned relative to the inferior articularsurface when the first end is secured to the first fixation member andthe attachment mechanism is in the locked configuration.
 18. The systemof claim 16, wherein the attachment mechanism further comprises alocking member, wherein the locking member is actuatable to exert forceagainst the second end to restrict rotation of the second end relativeto the articular surface to provide the locked configuration.
 19. Thesystem of claim 16, wherein the locking member is shaped to be receivedin the first opening such that the locking member is entirely positionedwithin the capture feature when the attachment mechanism is in thelocked configuration.
 20. The system of claim 16, wherein the entireinferior articular surface is positioned laterally of the sagittal planeof the first and second vertebrae when the attachment mechanism is inthe locked configuration.
 21. The system of claim 13, furthercomprising: a second fixation member implantable in the first vertebra;and a superior facet joint implant securable to the second fixationmember, the superior facet joint implant comprising a superior articularsurface shaped to replace a natural superior articular surface of thefirst vertebra.
 22. The system of claim 13, wherein the inferior strutfurther comprises a tapered portion positioned between the first end andthe second end.
 23. The system of claim 13, wherein the inferior strutis radially symmetrical such that a cross-section taken orthogonal tothe longitudinal axis of the inferior strut at any location along theinferior strut is circular in shape.
 24. The system of claim 13, whereinthe inferior facet joint implant is one of a left inferior facet jointimplant configured to be implanted on the left lateral side of thesecond vertebra to replace the natural left inferior facet joint, and aright inferior facet joint implant configured to be implanted on theright lateral side of the second vertebra to replace the natural rightinferior facet joint.
 25. The system of claim 13, wherein the firstfixation member comprises an adjustable configuration in which the firstend of the inferior strut is connected to the first fixation member in amanner that permits relative rotation between the first end and thefirst fixation member about three orthogonal axes, and a lockedconfiguration in which the first end is rigidly secured to the firstfixation member.
 26. A system for replacing at least a portion of anatural facet joint that controls motion between a first vertebra and asecond vertebra, the system comprising: a first fixation memberimplantable in a pedicle of the second vertebra; and an inferior facetjoint implant comprising: an inferior articular surface shaped toreplace a natural inferior facet articular surface of the secondvertebra; an inferior strut formed separately from the inferiorarticular surface, the inferior strut comprising a first end and asecond end, wherein the first end is securable to the fixation member;and an attachment mechanism comprising a capture feature shaped toreceive the second end, the attachment mechanism further comprising anadjustable configuration in which the inferior articular surface isconnected to the second end in a manner that permits relative rotationbetween the inferior articular surface and the second end about threeorthogonal axes, and a locked configuration in which the inferiorarticular surface is rigidly secured to the second end, wherein theinferior articular surface is lateral to the sagittal plane of the firstand second vertebrae when the attachment mechanism is in the lockedconfiguration.
 27. The system of claim 26, wherein the capture featureis formed monolithically with the inferior articular surface.
 28. Thesystem of claim 26, wherein the capture feature is medially positionedrelative to the inferior articular surface when the first end is securedto the fixation member and the attachment mechanism is in the lockedconfiguration.
 29. The system of claim 26, wherein the attachmentmechanism further comprises a locking member, wherein the locking memberis actuatable to exert force against the second end to restrict rotationof the second end relative to the articular surface to provide thelocked configuration.
 30. The system of claim 29, wherein the lockingmember is shaped to be received in the capture feature such that thelocking member is entirely positioned within the capture feature whenthe attachment mechanism is in the locked configuration.
 31. The systemof claim 26, further comprising: a second fixation member implantable inthe first vertebra; and a superior facet joint implant securable to thesecond fixation member, the superior facet joint implant comprising asuperior articular surface shaped to replace a natural superiorarticular surface of the first vertebra.
 32. The system of claim 26,wherein the inferior strut further comprises a tapered portionpositioned between the first end and the second end.
 33. The system ofclaim 26, wherein the inferior strut is radially symmetrical such that across-section taken orthogonal to the longitudinal axis of the inferiorstrut at any location along the inferior strut is circular in shape. 34.The system of claim 26, wherein the inferior facet joint implant is aleft inferior facet joint implant configured to be implanted on the leftlateral side of the second vertebra to replace the natural left inferiorfacet joint.
 35. The system of claim 26, wherein the first fixationmember comprises an adjustable configuration in which the first end ofthe inferior strut is connected to the first fixation member in a mannerthat permits relative rotation between the first end and the firstfixation member about three orthogonal axes, and a locked configurationin which the first end is rigidly secured to the first fixation member.36. The system of claim 26, wherein the second end comprises a spherewith an uninterrupted hemispherical surface.